Python is a very popular coding language. Little people use it for serious
projects, but it is still useful to learn. It was created in 1991 by
Guido van Rossum
.
 For example, the
python language uses a :
and indented code, while javascript
and others generally use {}
and indented code.
Lets create a python 3 repl, and call it Hello World. Now you have a blank file called main.py. Now let us write our first line of code:
helloworld.py
print('Hello world!')
Brian Kernighan actually wrote the first “Hello, World!” program as part of the documentation for the BCPL programming language developed by Martin Richards.
Now, press the run button, which obviously runs the code. If you are not using replit, this will not work. You should research how to run a file with your text editor.
If you look to your left at the console where hello world was just
printed, you can see a >
, >>>
, or
$
depending on what you are using. After the prompt, try
typing a line of code.
Python 3.6.1 (default, Jun 21 2017, 18:48:35)
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
> print('Testing command line')
Testing command line
> print('Are you sure this works?')
Are you sure this works?
>
The command line allows you to execute single lines of code at a time. It is often used when trying out a new function or method in the language.
Another cool thing that you can generally do with all languages, are
comments. In python, a comment starts with a #
. The computer
ignores all text starting after the #
.
shortcom.py
# Write some comments!
If you have a huge comment, do not comment all the 350
lines, just put '''
before it, and '''
at the
end. Technically, this is not a comment but a string, but the computer
still ignores it, so we will use it.
longcom.py
'''
Dear PYer,
I am confused about how you said you could use triple quotes to make
SUPER
LONG
COMMENTS
!
I am wondering if this is true,
and if so,
I am wondering if this is correct.
Could you help me with this?
Thanks,
Random guy who used your tutorial.
'''
print('Testing')
Unlike many other languages, there is no var
,
let
, or const
to declare a variable in python.
You simply go name = 'value'
.
vars1.py
x = 5
y = 7
z = x*y # 35
print(z) # => 35
Remember, there is a difference between integers and strings.
Remember: String = ""
. To convert between these two,
you can put an int in a str()
function, and a string in a
int()
function. There is also a less used one, called a
float. Mainly, these are integers with decimals. Change them using the
float()
command.
vars2.py
x = 5
x = str(x)
b = '5'
b = int(b)
print('x = ', x, '; b = ', str(b), ';') # => x = 5; b = 5;
Instead of using the ,
in the print function, you can put a
+
to combine the variables and string.
There are many operators in python:
+
-
/
*
%
//
**
+=
-=
/=
*=
simpleops.py
x = 4
a = x + 1
a = x - 1
a = x * 2
a = x / 2
You should already know everything shown above, as it is similar to other languages. If you continue down, you will see more complicated ones.
complexop.py
a += 1
a -= 1
a *= 2
a /= 2
The ones above are to edit the current value of the variable.
Sorry to JS users, as there is no i++;
or anything.
Fun Fact:
The python language was named after Monty Python.
If you really want to know about the others, view Py Operators
Like the title?
Anyways, a '
and a "
both indicate a string, but
do not combine them!
quotes.py
x = 'hello' # Good
x = "hello" # Good
x = "hello' # ERRORRR!!!
slicing.py
You can look at only certain parts of the string by slicing it, using
[num:num]
.
The first number stands for how far in you go from the front, and the
second stands for how far in you go from the back.
x = 'Hello everybody!'
x[1] # 'e'
x[-1] # '!'
x[5] # ' '
x[1:] # 'ello everybody!'
x[:-1] # 'Hello everybod'
x[2:-3] # 'llo everyb'
Here is a list of functions/methods we will go over:
.strip()
len()
.lower()
.upper()
.replace()
.split()
I will make you try these out yourself. See if you can figure out how they work.
strings.py
x = " Testing, testing, testing, testing "
print(x.strip())
print(len(x))
print(x.lower())
print(x.upper())
print(x.replace('test', 'runn'))
print(x.split(','))
Good luck, see you when you come back!
Input is a function that gathers input entered from the user in the command line. It takes one optional parameter, which is the users prompt.
inp.py
print('Type something: ')
x = input()
print('Here is what you said: ', x)
If you wanted to make it smaller, and look neater to the user, you could do…
inp2.py
print('Here is what you said: ', input('Type something: '))
Running:
inp.py
Type something:
Hello World
Here is what you said: Hello World
inp2.py
Type something: Hello World
Here is what you said: Hello World
Python has created a lot of functions that are located in other .py files.
You need to import these modules to gain access to the,,
You may wonder why python did this. The purpose of separate modules is to
make python faster. Instead of storing millions and millions of functions,
, it only needs a few basic ones. To import a module, you must write
input <modulename>
. Do not add the .py extension to the
file name. In this example , we will be using a python created module
named random.
module.py
import random
Now, I have access to all functions in the random.py file. To access a
specific function in the module, you would do
<module>.<function>
. For example:
module2.py
import random
print(random.randint(3,5)) # Prints a random number between 3 and 5
Pro Tip:
Dofrom random import randint
to not have to dorandom.randint()
, justrandint()
To import all functions from a module, you could dofrom random import *
Loops allow you to repeat code over and over again. This is useful if you want to print Hi with a delay of one second 100 times.
for
Loop
The for loop goes through a list of variables, making a seperate variable
equal one of the list every time.
Let’s say we wanted to create the example above.
loop.py
from time import sleep
for i in range(100):
print('Hello')
sleep(.3)
This will print Hello with a .3 second delay 100 times. This is just one way to use it, but it is usually used like this:
loop2.py
import time
for number in range(100):
print(number)
time.sleep(.1)
while
Loop
The while loop runs the code while something stays true. You would put
while <expression>
. Every time the loop runs, it
evaluates if the expression is True. It it is, it runs the code, if not it
continues outside of the loop. For example:
while.py
while True: # Runs forever
print('Hello World!')
Or you could do:
while2.py
import random
position = '<placeholder>'
while position != 1: # will run at least once
position = random.randint(1, 10)
print(position)
The if statement allows you to check if something is True. If so, it runs the code, if not, it continues on. It is kind of like a while loop, but it executes only once. An if statement is written:
if.py
import random
num = random.randint(1, 10)
if num == 3:
print('num is 3. Hooray!!!')
if num > 5:
print('Num is greater than 5')
if num == 12:
print('Num is 12, which means that there is a problem with the python language, see if you can figure it out. Extra credit if you can figure it out!')
Now, you may think that it would be better if you could make it print only
one message. Not as many that are True. You can do that with an
elif
statement:
elif.py
import random
num = random.randint(1, 10)
if num == 3:
print('Num is three, this is the only msg you will see.')
elif num > 2:
print('Num is not three, but is greater than 1')
Now, you may wonder how to run code if none work. Well, there is a simple
statement called else:
else.py
import random
num = random.randint(1, 10)
if num == 3:
print('Num is three, this is the only msg you will see.')
elif num > 2:
print('Num is not three, but is greater than 1')
else:
print('No category')
So far, you have only seen how to use functions other people have made.
Let use the example that you want to print the a random number between 1
and 9, and print different text every time.
It is quite tiring to type:
Characters: 389
nofunc.py
import random
print(random.randint(1, 9))
print('Wow that was interesting.')
print(random.randint(1, 9))
print('Look at the number above ^')
print(random.randint(1, 9))
print('All of these have been interesting numbers.')
print(random.randint(1, 9))
print("these random.randint's are getting annoying to type")
print(random.randint(1, 9))
print('Hi')
print(random.randint(1, 9))
print('j')
Now with functions, you can seriously lower the amount of characters:
Characters: 254
functions.py
import random
def r(t):
print(random.randint(1, 9))
print(t)
r('Wow that was interesting.')
r('Look at the number above ^')
r('All of these have been interesting numbers.')
r("these random.randint's are getting annoying to type")
r('Hi')
r('j')
This video should help you get up and running with Python 3
Installing Python is really a cakewalk. Search for “Python download” on www.google.com. Download the installable and install it.
A quick word of caution on Windows
Once you have installed Python, you can launch the Python Shell.
Command to launch Python 3 is different in Mac.
python3
python
You can type code in python shell and code as well!
You can use print(5*4)
, and it shows 20
.
You can execute the code, and the shell would immediately give you output.
Using the the Python Shell is an awesome way to learn Python.
Most programmers find programming a lot of fun, and besides, it also gets their work done.
Programming mainly involves problem solving, where one makes use of a computer to solve a real world problem.
During our journey here, we will approach programming in a very different way. We will not only introduce you to the Python language, but also help you pick up essential problem solving skills.
As a programmer, you need to be able to look at a problem, and identify the important programming concepts relevant to solving it. Finally, you need to be able to use the language features and syntax, to express your solution on the computer. While all this looks complex, we want to make it easy for you. Together, we will tackle a variety of programming challenges, using these same steps. We will start with simple challenges (such as a Multiplication Table), and gradually increase the difficulty level over the duration of this book.
Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.
Once you get over these initial steps, your experience will become more and more enjoyable.
Are you ready for your first programming challenge? Let’s get going now! We wish you all the best.
In this step, we:
Our first programming challenge aims to do, what every kid does in math class: read out a multiplication table. We now want to give this task to the computer. Here is the statement of our problem:
5
, with entries from
1
to 10
.
The display needs to be:
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50
This is the challenge. For convenience, let’s give it a label, say PMT-Challenge. What would be the important concepts we need to learn, to solve this challenge? The following list of concepts would be a good starting point:
In the rest of this chapter, we will introduce these concepts to you, one-by-one. We will also show you how learning each concept, takes us closer to a solution to PMT-Challenge.
In this step, we:
Typically when we do programming, we have problems. Solving the problem typically need a step-by -step approach. Common sense tells us that to solve a complex problem, we break it into smaller parts, and solve each part one by one. Here is how any good programmer worth her salt, would solve a problem:
The PMT-Challenge is no different! Now how do we break it down,
and where do we really start? Once again, your common sense will reveal a
solution. As a first step, we could get the computer to calculate say,
5 * 3
. The second thing we can do, is to try and print the
calculated value, in a manner similar to 5 * 3 = 15
. Then, we
could repeat what we just did, to print out all the entries of the
5
multiplication table. Let’s put it down a little more
formally:
Here is how our draft steps look like
5 * 3
and print result as 15
5 * 3 = 15
(15
is result of previous
calculation)
1
to 10
)
Let’s start with that kind of a game plan, and see where it takes us.
In this step, we:
Let’s focus on solving the first sub-problem of PMT-Challenge,
the numeric computation. We want the computer to calculate
5 * 5
for example, and print 25
for us. How do
we get it to do that? That’s what we would be looking at in this step.
Launch up Python shell. We want to calculate 5 * 5
. How do we
do that?
Using our knowledge of school math, let’s try 5 X 5
.
>>> 5 X 5
File "< stdin >", line 1
5 X 5
^
SyntaxError: invalid syntax
The Python Shell hits back at us, saying “invalid syntax”. This is how Python complains, when it doesn’t fully understand the code you type in. Here, it says our code has a “SyntaxError”.
The reason why it complains, is because ‘X
’ is not a valid
operator in Python.
The way you can do multiplication is by using the ‘*
’
operator .
“5 into 5” is achieved by the code 5 * 5
, and you
can see the result 25
being printed. Similarly,
5 * 6
gives us 30
.
>>> 5 * 6
30
There are a wide range of other operators in Python:
5 + 6
gives a result of 11
.5 - 6
leads to -1
.
>>> 5 + 6
11
>>> 5 - 6
-1
10 / 2
, gives an output of 5.0
. There is one
interesting operator, **
. Let’s try 10 ** 3
. We
ran this code, and the result we get is 1000
. Yes you guessed
right, the operator performs “to the power of”. “10
to the
power of 3
” is 10 * 10 * 10
, or
1000
.
>>> 10 / 2
5.0
>>> 10 ** 3
1000
Another interesting operator is %
, called “modulo”,
which computes the remainder on integer division. If we do
10 % 3
, what is the remainder when 10
is divided
by 3
? 3 * 3
is 9
, and
10 - 9
is 1
, which is what
%
returns in this case.
Let’s look at some terminology:
5 * 5
,
5 * 6
and 5 - 6
are all expressions.
An expression is composed of operators and
operands.
5 * 6
, the two values 5
and
6
are called operands, and the *
operator
operates on them.
5
and 6
are
literals, because those are constants which cannot be
changed.
The cool thing about Python, is that you can even have expressions with
multiple operators. Therefore, you can form an expression with
5 + 5 + 5
, which evaluates to 15
. This is an
expression which has three operands, and two +
operators. You
can even have expressions with different types of operators, such as in
5 + 5 * 5
.
>>> 5 + 5 + 5
15
>>> 5 + 5 * 5
30
Try and play around with the expressions, and understand the output which results.
In this step, we:
At this stage, your smile tells us that you enjoy evaluating Python expressions. What if we tickle your mind a bit, to make sure it hasn’t fallen asleep? Here is your first programming exercise.
You need to solve these problems by yourself. If you are able to work them out, that’s fantastic! But if not, that’s part of the learning process.
>>> 24 * 60
1440
We wanted to calculate the number of minutes in a day. How do we do that? Think about this…
24
.60
.24 * 60
, which is 1440
.
>>> 24 * 60 * 60
86400
How many seconds are there in a day?
24
.60
, and60
as well.24 * 60 * 60
, or 86400
.In this step, we:
Let’s look at a few puzzles related to expressions, in this step. Before that, let’s revise some of the terminology we had learned earlier.
5 + 6 + 10
is an example of an expression. In this
expression, 5
, 6
and 10
are
operands. The +
here is the operator. You can have multiple
operators in an expression. We also did mention that the operands, namely
10
, 6
and 5
, are literals. Their
values will not change.
Here are a few puzzles coming up, to explore aspects of expressions.
Think about what would happen when you do something of this kind:
5 $ 2
. You’re right, it would throw a
SyntaxError
. When Python does not understand the code you
type in, it reports an error. Here, the expression we’re typing is
5 $ 2
, which does not make sense to Python, hence the
SyntaxError
.
>>> 5 $ 2
File "< stdin >", line 1
5 $ 2
^
SyntaxError: invalid syntax
>>> 5$2
File "< stdin >", line 1
5 $ 2
^
SyntaxError: invalid syntax
Let’s say we type in 5+6+10
, without any spaces between the
operands, and the operators. What do you think will happen? Surprisingly,
the Python Shell does calculate the value!
>>> 5+6+10
21
In an expression, using spaces makes it easier for you to read it, but
it’s not mandatory. 5 + 6 + 10
is easier to read than
5+6+10
, but does not make any difference to the Python
compiler.
The next puzzle tries to evaluate 5 / 2
, which is “5
divided by 2
”. What would be the output? 2.5
.
>>> 5/2
2.5
If you’re coming from other programming languages like Java or C, this
might be a surprising result. If you try this in Java for instance, you
would get 2
as the output. Note that even though both
operands are integers, the result of the /
operation is a
floating point value, 2.5
. Python does what is expected by a
programmer!
The puzzle after that tries to play with 5 + 5 * 6
. What
would be the result of this expression? Will it be 5 + 5
or
10
, then 10 * 6
, which is 60
? Or,
will it be 5
plus 5 * 6
, which is
5
+ 30
, that’s 35
?
>>> 5 + 5 * 6
35
The correct result is 35
.
Python decides this is based on the precedence of operators.
Operators in Python are divided into two sets as follows:
**
, *
, /
and %
have
higher precedence, or priority.
+
and -
have a lower precedence.
Sub-expressions involving operators from {*
, /
,
%
, **
} are evaluated before those involving
operators from {+
, -
}
Let’s try another small puzzle on precedence, with 5 - 2 * 2
.
What would be the result of this? Will it be 6
, or
1
? It’s 1
, because *
has a higher
precedence than -
. Thus 2 * 2
is 4
,
and 5 - 4
gives us 1
.
>>> 5 - 2 * 2
1
Let’s say we want to execute 5 - 2
, to give an output of
2
. How do we change the operator precedence?
You cannot really change the precedence, but you can add parentheses to group sub-expressions differently.
>>> (5 - 2) * 2
6
>>> 5 - ( 2 * 2 )
1
Parentheses have the highest precedence in Python, and can be used to
override operator precedence. (5 - 2)
gets calculated first,
and the final result of the expression is 6
.
A positive thing about using parentheses is, that it makes expressions
more readable. So even in situations such as 5 - 2 * 2
, where
we know the result according to precedence, adding parentheses is good.
In this step, we went about solving a few puzzles about expressions, touching concepts such as:
SyntaxError
for incorrect operatorsIn the previous step, we learned how to use expressions to compute values. In this step, let’s see how we can actually print multiplication table entries, that are readable by the user.
How do we go about printing a complete multiplication table entry? We want
to print text such as 5 * 6 = 30
. But trying to do so, as we
know it, gives us a SyntaxError
. Clearly, there is a
different way to print text, as compared to an expression.
>>> 5 * 6 = 30
File "<stdin>", line 1
SyntaxError: can't assign to operator
Let’s first try to print a simple piece of text, Hello
.
Typing in this piece of code directly on Python Shell also gives us an
error.
>>> Hello
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Hello' is not defined
Only expressions work that way, and Hello
is not really an
expression.
"Hello"
is typically called a string, and
represents the text of letters 'H'
, 'e'
,
'l'
, 'l'
, 'o'
.
"Hello"
is hence different from the number 5
.
There are a number of in-built functions in Python to help print strings.
One of these is the print()
function. Can you just say
print Hello
?
>>> print Hello
File "<stdin>", line 1
print Hello
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(Hello)?
The Python compiler gives you an error, that says “missing parentheses”.
Will print(Hello)
work?
>>> print (Hello)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Hello' is not defined
Nope! Again, this one failed because you need to indicate that
"Hello"
is a string.
How do I indicate that "Hello"
is a string? By putting it
within double quotes.
Let’s try print ("Hello")
>>> print ("Hello")
Hello
>>> print("Hello")
Hello
print("Hello")
finally results in "Hello"
being
printed out. To be able to print "Hello"
, the things we need
to do are:
What we have written here is called a statement, a simple
piece of code to execute. As part of this statement, we are
calling a function, named
print()
.
What exactly are we trying to print?
The text "Hello"
, which is called a
parameter or argument, to
print()
.
Now let’s get back to what we wanted to do, which is to print
5 * 6 = 30
. The most basic version would be something of this
kind, print("5 * 6 = 30")
. Here, we are passing the entire
value in the form of a string.
>>> print("5 * 6 = 30")
5 * 6 = 30
This prints the text on the console, as-is. The thing you need to
understand here is, we aren’t really calculating 30
using the
formula 5 * 6
, but directly putting text 30
in
here. That’s called hard-coding.
In a later step, we will look at how to actually calculate the value and pass it in.
In this step, we:
print()
function, that is used to print
text in Python.
"5 * 6 = 30"
on the console,
by hard-coding values in a string
In the previous step, we learned how to print 5 * 6 = 30
. It
was not a perfect solution, because we hard-coded everything. we used an
in-built function named print()
, passed a string to it, and
invoked the method.
In this step, let’s look at a number of puzzles related to in-built methods, their parameters, and strings in general.
For example, let’s do print("5 * 6")
, as in the previous
step. What does this code result in?
>>> print("5*6")
5*6
>>> print('5*6')
5*6
It just prints the string "5 * 6"
.
Let’s say we try the code print(5 * 6)
,
>>> print(5*6)
30
Without the double quotes, 5 * 6
is an expression. What will
be the output? 30
.
If you call print()
with an expression argument, it prints
the value of the expression. However, when we pass something within double
quotes, it becomes a piece of text, printed as-is.
An interesting thing to note is, that in Python you can use either
double-quotes ("
and "
), or single-quotes ('
and '
) with text values.
Let’s look at a few other in-built methods within Python.
Consider abs()
(which stands for absolute value), a method
that accepts a numeric value. You can use abs(10.5)
, passing
10.5
as a value to it, and it prints the absolute value of
10
.
>>> abs 10.5
File "<stdin>", line 1
abs 10.5
^
SyntaxError: invalid syntax
>>> abs(10.5)
10.5
If you pass in a string value, will it work? It complains, “abs()
function will not work with a string, it only works with numeric values”.
>>> abs("10.5")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: bad operand type for abs(): 'str'
Let’s say you want to use a function that computes “to the power of”, for
instance “2
to the power of 5
”. In Python,
there’s an in-built function named pow()
, which does just
what we need. To pow()
, you can pass two parameters and
calculate the result. How do you do that?
Will this work: pow 2 5
? No, not at all. This code does not
work as well: pow(2 5)
. pow(2, 5)
is the correct
syntax.
>>> pow 2 5
File "<stdin>", line 1
pow 2 5
^
SyntaxError: invalid syntax
>>> pow(2 5)
File "<stdin>", line 1
pow(2 5)
^
SyntaxError: invalid syntax
>>> pow(2, 5)
32
You’ll see that 32
is printed.
Let’s see another example, “10
to the power of
3
”. pow(10,3)
is the alternative to saying
10 ** 3
. This gives us 1000
, similar to how
pow()
would.
>>> pow(10, 3)
1000
>>> 10 ** 3
1000
max()
returns maximum in a set of numbers.min()
function returns the minimum value.
>>> max(34, 45, 67)
67
>>> min(34, 45, 67)
34
These are some of the in-built functions in Python, and we saw how to call the in-built functions by passing in a varied number of parameters.
Python is case sensitive. So let’s say I want of calculate
pow(2,5)
. So this would give me 32
. Now, what if
I say capital 'P'
instead of small 'p'
here?
Pow(2,5)
would lead to an error.
>>> pow(2,5)
32
>>> Pow(2,5)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Pow' is not defined
The only things not case-sensitive in Python, are string values. Earlier
we saw that the code print("Hello")
displays the text
"Hello"
. Inside a string, the text can be in any case. Hence,
print("hello")
displays "hello"
,with a small
'h'
.
>>> print("Hello")
Hello
>>> print("hello")
hello
>>> print("hellO")
hellO
>>> print ( "hellO" )
hellO
However inside your code, you need to be very particular about the case of function names, class names, variable names, and the like.
In your code, whitespace does not really matter. You can add space here and here, and you would still get the same output. However, in case of strings, whitespace does matter.
If we say print("hellO World")
, it would print
"hellO World"
, with a space in between. And if you do
print("hellO World")
with three spaces, it would print the
same. In expressions, white-space does not affect the output.
>>> print ( "hellO World" )
hellO World
>>> print ( "hellO World" )
hellO World
The last thing we want to look at, is an escape sequence.
Let’s say you want to print a double quote, "
, in the code.
If we were to do this: print("Hello"")
, what would happen?
The compiler says error!
>>> print("Hello"")
File "<stdin>", line 1
print("Hello"")
^
SyntaxError: EOL while scanning string literal
If you want to print a "
inside a string, use an escape
sequence. In Python, the symbol '\'
is used as an
escape character. On using '\'
adjacent to
the "
, it prints Hello"
(notice the trailing
"
). We have used the '\'
to
escape the "
, by forming an
escape sequence \"
.
>>> print("Hello\"")
Hello"
>>>
The other reason why you would want to use a '\'
is to print
a <NEWLINE>
. If you want to print
"Hello World"
, but with "Hello"
on one line and
"World"
on the next, '\n'
is the escape sequence
to use.
>>> print("Hello\nWorld")
Hello
World
The other important escape sequence is '\t'
, which prints a
<TAB>
in the output. When you do
print("Hello\tWorld")
, you can see the tab-space between
"Hello"
and "World"
.
>>> print("Hello\tWorld")
Hello World
Another useful escape sequence is \\
. If you want to print a
\
, then use the sequence \\
. You would see
that it prints Hello\World
. Think about what would happen if
we put six \
. Yes you’re right! It would print this string:
"\\\"
.
>>> print("Hello\\World")
Hello\World
>>> print("Hello\\\\\\World")
Hello\\\World
One of the things with Python is, it does not matter whether you use
double quotes or single quotes to enclose strings. There are some
interesting, and useful ways of using a combination of both, within the
same string. Have a look at this call: print("Hello'World")
,
and notice the output we get. In a similar way, the following code will be
accepted and run by the Python system: print('Hello"World')
.
>>> print('Hello"')
Hello"
>>> print("Hello'World")
Hello'World
>>> print("Hello\"World")
Hello"World
>>> print("Hello\"World")
Hello"World
The above two examples can be used as a tip by newbie programmers when they form string literals, and want to use them in their code:
In this step, we:
print()
function to display expressions and strings
In the previous step, we learned how to print a hard-coded string, such as
"5 * 6 = 30"
.
In this step, let’s try to replace the hard-coded 30
with a
computed value.
Let’s start with a simple scenario. Let’s say we want to place that calculated value within a string, and display it. How do we do that?
format()
method can be used to print formatted text.
Let’s see an example:
>>> print("VALUE".format(5*2))
VALUE
We were expecting 10
to be printed, but it’s actually
printing VALUE
.
How do we get 10
to be printed then?
>>> print("VALUE {0}".format(5*2))
VALUE 10
By having an open brace {
, closed brace }
, and
and by putting the index of the value between them. Here, the value is the
first parameter, and it’s index will be 0
.
"VALUE {0}"
is what we need.
Let’s take another example. Suppose to the format()
function,
we pass three values: 10
, 20
and
30
.
Typically when we count positions or indexes, we start from
0
.
To print the first value, you need to pass in an index of 0
.
To print the second value, pass an index of 1
.
>>> print("VALUE {0}".format(10,20,30))
VALUE 10
>>> print("VALUE {1}".format(10,20,30))
VALUE 20
>>> print("VALUE {2}".format(10,20,30))
VALUE 30
Now going back to our problem, we wanted to display
"5 * 6 = 30"
, but without hard-coding. Instead of
30
, we want the calculated value of 5 * 6
.
>>> print("5 * 6 = 30".format(5,6,5*6))
5 * 6 = 30
Let replace "5 * 6 = 30"
with "5 * 6 = {2}"
.
2
is the index of parameter value 5*6
.
>>> print("5 * 6 = {2}".format(5,6,5*6))
5 * 6 = 30
Cool! Progress made.
Let’s replace 5 * 6
with the right indices -
{0} * {1}
.
>>> print("{0} * {1} = {2}".format(5,6,5*6))
5 * 6 = 30
The great thing about this, is now we can replace the values we passed to
print()
in the first place, without changing the indexes! So,
we can display results for 5 * 7 = 35
and
5 * 8 = 40
. We are now able to print 5 * 6 = 30
,
5 * 7 = 35
, 5 * 8 = 40
, and can do similar
things for other table entries as well.
>>> print("{0} * {1} = {2}".format(5,7,5*7))
5 * 7 = 35
>>> print("{0} * {1} = {2}".format(5,8,5*8))
5 * 8 = 40
>>> print("{0} * {1} = {2}".format(5,8,5*8))
5 * 8 = 40
In this step, we:
format()
function, and saw how to call it
within print()
format()
, and change the parameters we pass without
changing the code
In this step, let’s look at a few puzzles related to the format, and the print methods.
Let’s say we pass in additional values, such as: 5 * 8
,
5 * 9
and 5 * 10
. However, within the call to
format()
, we are only referring to the values at index
0
, index 1
and index 2
. The values
at indexes 3
and 4
are not used at all. What
would happen when we run the code?
>>> print("{0} * {1} = {2}".format(5,8,5*8,5*9,5*10))
5 * 8 = 40
Would this throw an error? No, it does not. You can see that the additional values which are passed in, are conveniently ignored.
Let’s say instead of passing in a value of 2
, we pass
4
. What would happen?
>>> print("{0} * {1} = {4}".format(5,8,5*8,5*9,5*10))
5 * 8 = 50
5 * 10
is the value at index 4
Now let’s take a different scenario. We remove all the parameters passed
to format()
. However, inside the call to
print()
, we continue to say {0} * {1} = {4}
. So
we are trying to print the value at index 4
, but are only
passing two values to the function format()
. What do you
think will happen?
>>> print("{0} * {1} = {4}".format(5,8))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: tuple index out of range
It says IndexError
, which means :“you are asking me to fetch
the value at index 4
, but only passing in two values. How can
I do what you want?”
Let’s look at a few more things related to other data types. We try to
format the following inside print()
:
{0} * {1} = {2}
, and would pass in 2.5
,
2
, and 2.5 * 2
. Here, 2
is an
integer value, but 2.5
is a floating point value. You can see
that it prints 2.5 * 2 = 5.0
. So this approach of formatting
values with print()
, works also with floating point data as
well.
>>> print("{0} * {1} = {2}".format(2.5,2,2.5*2))
2.5 * 2 = 5.0
Now, are there are other types of data that format()
works
with? Yes, strings can join the party.
Let’s say over here, we do:
print("My name is {0}".format("Ranga"))
. What would happen?
>>> print("My name is {0}".format("Ranga"))
My name is Ranga
Index 0
will be replaced with the first parameter to
format()
.
In this step, we:
format()
:
print()
print()
We are slowly making progress toward our main goal, which is to print the
5
multiplication table.
In the first statement, we are printing 5 * 1 = 5
, and then
changing the literals. To make it print 5 * 2 = 10
, we are
changing 1
to 2
. Next, we are changing
2
to 3
. How do we make it a little simpler, so
that our effort is reduced?
>>> print("{0} * {1} = {2}".format(5,1,5*1))
5 * 1 = 5
>>> print("{0} * {1} = {2}".format(5,2,5*2))
5 * 2 = 10
>>> print("{0} * {1} = {2}".format(5,3,5*3))
5 * 3 = 15
Let’s try a different approach.
What would happen if you replace 1
with index
,
and 5 * 1
with 5 * index
, and try to run it?
It gives an error! It says: “index is not defined”.
Let’s try and fix this, and execute index = 2
. What would
happen?
>>> index = 2
Aha! This compiles.
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 2 = 10
And this statement is printing 5 * 2 = 10
.
Let’s try something else. Let’s make index = 3
. What would
happen?
>>> index = 3
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 3 = 15
The same statement on being run, prints 5 * 3 = 15
.
How can you check the value that index
has? Just type in
index
.
>>> index
3
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 3 = 15
The index
symbol we have used here, is what is called a
variable.
In Python, it’s also called a name.
You can see that the value index
referring to, can change
over the duration of a program.
Initially, index
was referring to a value of 1
.
later, index
was referring to a value of 3
.
Now, think about how you would print the entire table. All that you need
to do, is start from 1
, execute the same statement with
print()
and format()
, to get output
5 * 1 = 5
. Next, Change the value of index to 2
,
and then print the same statement. Next, index = 3
, and print
the same statement again.
>>> index = 1
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 1 = 5
>>> index = 2
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 2 = 10
>>> index = 3
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 3 = 15
With the same statement
print("{0} * {1} = {2}".format(5,index,5*index))
, we are able
to print different values. The value of index
varies, but the
code remains the same!
Variables make the program much more easier to read, as well as more generic.
Let’s do a simple exercise with variables.
We want to create three variables a
, b
and
c
. Let’s initially give them some values, say a value of
5
to a
, 6
to b
and
7
to c
.
We want to get output of this kind: 5 + 6 + 7 = 18
, without
using the literal values.
You would want to use the values stored in the variables in
a
, b
and c
.
If you’re hard-coding, the way to do it is with
print("5 + 6 + 7 = 18")
.
>>> a = 5
>>> b = 6
>>> c = 7
>>> print("5 + 6 + 7 = 18")
5 + 6 + 7 = 18
>>> print("5 + 6 + 7 = 18".format(a,b,c,a+b+c))
5 + 6 + 7 = 18
The way you can do that is with code like this:
print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
.
>>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
5 + 6 + 7 = 18
How do you confirm we are accessing values stored in the variables?
Let’s change the values of a
, b
and
c
. Let’s make a = 6
, b = 7
, and
c = 8
. Execute same statement.
>>> a = 6
>>> b = 7
>>> c = 8
>>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
6 + 7 + 8 = 21
You can see the magic of variables at play here! Based on what values these variables are referring to, you can see that the output of the print statement changes.
In this step, we:
format()
function
In the previous step, we were introduced to the concept of variables in Python.
We will start with looking at a few puzzles.
What if I try to refer to a variable which is not yet created?
>>> count
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'count' is not defined
>>> print(count)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'count' is not defined
Before using a variable, you need to have it assigned a value. If you have
not defined a variable before, then you cannot use it. Consider
print(count)
, it does not know what count is. So it would
throw an error, saying: “count
is not defined, I have no idea
what count is.”
Once you assign a value to a variable, you can use it.
>>> count = 4
>>> print(count)
4
The statement count = 4
where we are creating a variable
named count
for the first time, is called a
variable definition.
This is the first time you’re referring to a variable, and assigning a value to it.
Python will create a variable in its memory.
Variable names are case sensitive. count
and
Count
are not the same thing.
>>> Count
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Count' is not defined
>>> count
4
There are rules to follow while naming variables.
All variable names should either start with an alphabet , or an underscore
_
. count
, _count
are valid.
1count
is invalid.
>>> 1count = 5
File "<stdin>", line 1
1count = 5
^
SyntaxError: invalid syntax
>>> count = 5
>>> _count = 5
>>> 1count
File "<stdin>", line 1
1count
^
SyntaxError: invalid syntax
>>> 2count
File "<stdin>", line 1
2count
^
SyntaxError: invalid syntax
After the first symbol, you can also use a numeral in variable names.
>>> c12345 = 5
To summarize the rules for naming variables.
In this step, we:
In this step, we will look at an important concept in Python, called
assignment. In previous steps, we created variables, like
i = 5
.
You can create other variables using whatever value i
is
referring to. If we say j = i
, what would happen?
>>> i = 5
>>> j = i
>>> j
5
j
would start referring to the same value that
i
is referring to. This statement is called an
assignment.
Let’s try j = 2 * i
.
>>> j = 2 * i
>>> j
10
j
refers to a value of 10
=
has a different meaning in programming compared to
mathematics.
In mathematics, When we execute j = i
, it means
j
and i
are equal.
In prgramming, the value of the expression on right hand side is assigned to the variable on the right hand side. Can you use a constant on the left hand side of an assignment? The answer is “No”!
>>> 5 = j
File "<stdin>", line 1
SyntaxError: can't assign to literal
The Python Shell throws an error, saying “Can’t assign to literal”, as
5
is a literal.
Let’s create a couple of variables. num1 = 5
and
num2 = 3
. We would want to add these and create a fresh
variable. Let’s say the name of the variable is sum
.
>>> num1 = 5
>>> num2 = 3
>>> sum = num1 + num2
>>> sum
8
Create 3 variables a
, b
and c
with
different values and calculate their sum.
>>> a = 5
>>> b = 6
>>> c = 7
>>> sum = a + b + c
>>> sum
18
We have just seen the mechanics of how assignment works in Python.
In this step, we:
=
) operator
Until now, we have been using the format()
method to format
and print values. Let’s see a better approach to printing values.
This is the approach we used until now.
>>> a = 1
>>> b = 2
>>> c = 3
>>> sum = a + b + c
>>> print("{0} + {1} + {2} = {3}".format(a, b, c ,sum))
1 + 2 + 3 = 6
Python has the concept of formatted strings. The syntax to use a formatted
string is very simple - f""
.
If we want to print the value of a variable a
, we can use
{a}
in the text.
>>> print(f"")
>>> print(f"value of a is {a}")
value of a is 1
>>> print(f"value of b is {b}")
value of b is 2
The variable within braces is replaced by its value.
You can use expressions in a formatted string. Example below uses
{a+b}
.
>>> print(f"sum of a and b is {a + b}")
sum of a and b is 3
This feature was introduced in a Python 3 release.
Let’s get back to the original problem we wanted to solve: printing
5 + 6 + 7 = 18
, using formatted strings.
>>> print(f"{a} + {b} + {c} = {sum}")
1 + 2 + 3 = 6
You can see how easy it turns out to be!
We want to print the 5
-table from
5 * 1 = 5
onward, until we reach to 5 * 10 = 50
.
The best solution we have right now, is shown below:
>>> index = 1
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 1 = 5
>>> index = 2
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 2 = 10
>>> index = 3
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 3 = 15
>>> index = 4
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 4 = 20
Can we do something, to make sure that the code remains the same all the
time, but the index
value gets updated?
>>> index = index + 1
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 5 = 25
>>> index = index + 1
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 6 = 30
>>> index = index + 1
>>> print("{0} * {1} = {2}".format(5,index,5*index))
5 * 7 = 35
We used index = index + 1
to increment
index
value.
If we execute these same two statements again and again, we can print the entire table! This is exactly what loops help us do: execute the same statements repeatedly.
The simplest loop available in Python is the for loop.
When we run a for
loop, we need to specify the range of
values - 1
to 10
or 1
to
20
, and so on. range()
function helps us to
specify a range of values.
>>> range(1,10)
range(1, 10)
The syntax of the for
loop is:
for i in range(1, 10): ...
. Here, i
is the name
of the control variable. In Python, you need to put a
colon, ‘:
’, and in the next line give indentation.
>>> for i in range(1,10):
... print(i)
...
1
2
3
4
5
6
7
8
9
You would see that it prints from 1
to 9
.
When we run a loop in range(1, 10)
, 1
is
inclusive and 10
is exclusive.The
loop runs from 1
to the value before 10
, which
is 9
.
The leading whitespace before print(i)
is called
indentation. We’ll talk about indentation later, when we
talk about puzzles related to the for
loop.
How can you extend this concept to solving our PMT-Challenge problem?
>>> print(f"{5} * {index} = {5*index}")
5 * 7 = 35
What we were doing earlier, was calling print()
with a
formatted string. Now we want to print this statement for different values
of i
.
How can you do that?
Let’s start with a simple example.
>>> for i in range(1,11):
... print(f"{i}")
...
1
2
3
4
5
6
7
8
9
10
print(f"{i}")
prints the value of i.
Now, how do we get it to print 5 * 1 = 5
to
5 * 10 = 50
?
>>> for i in range(1,11):
... print(f"5 * {i} = {5 * i}")
...
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50
>>> 5 * 4 * 50
1000
print(f"5 * {i} = {5 * i}")
prints a specific multiple of 5.
In a previous step, we took a major step in programming. We wrote our first for loop with Python. In this step, let’s try a few puzzles to understand the for loop even further.
The syntax of the for loop we looked at earlier was:
for i in range(1, 10):
print(i)
Let’s say we write a for
loop, but don’t give a
:
after the range()
method, to close the first
line. What would happen?
>>> for i in range(1,10)
File "<stdin>", line 1
for i in range(1,10)
^
SyntaxError: invalid syntax
Invalid syntax. A :
is mandatory within the
for
loop syntax.
Let’s provide a :
and in the next line, use
print(i)
without space before it (without indentation).
>>> for i in range(1,10):
... print(i)
File "<stdin>", line 2
print(i)
^
IndentationError: expected an indented block
Most other programming languages use open brace {
and closed
brace }
as delimiters in a for
loop. However,
Python uses indentation to identify which code is part of a
for
loop, and which is not. So if we are writing the body of
a for
loop, we must use indentation, and leave atleast a
single <SPACE>
.
>>> for i in range(1,10):
... print(i)
...
1
2
3
4
5
6
7
8
9
How do we execute two lines of code as part of the for
loop?
>>> for i in range(1,10):
... print(i)
... print(2*i)
...
1
2
2
4
3
6
4
8
5
10
6
12
7
14
8
16
9
18
We are indenting both statements with a space - print(i)
and
print(2*i)
.
When for loop has only one line of code, you can specify it right after
the :
>>> for i in range(2,5): print(i)
...
2
3
4
However, this is not considered to be a good programming practice. Even
though you may want to execute just one statement in a
for
loop, indentation on a new line is recommended.
Another best practice is to use four <SPACE>
s for
indentation, instead of just two. This would give clear indentation of the
code.
>>> for i in range(2,5):
... print(i)
...
2
3
4
Anybody who looks at the code immediately understands that this
print()
is part of the for
loop.
Let’s say you only want to print the odd numbers till 10
,
which are 1
, 3
, 5
,
7
and 9
. The range()
function
offers an interesting option.
>>> for i in range (1,11,2):
... print(i)
...
1
3
5
7
9
In for i in range(1, 11, 2)
, we pass in a third argument,
called a step. After each iteration, the value of
i
is increment by step
.
In this step, we:
for
loop, which lay
emphasis on the following aspects of for:
range()
function
In the previous step, after initially exploring the Python
for
loop, we looked at a number of puzzles.
In this step, let’s look at a few exercises.
Instead of starting with 1
, we need to start with
2
. Each time, i
it would be incremented by
2
, and 2 4 6 8 and 10
would be printed.
>>> for i in range (2,11,2):
... print(i)
...
2
4
6
8
10
We would want to print the numbers in reverse. Think about how you would
do that using the range()
function. We’d want go from
10
, 9
, 8
, and so on up to
1
.
>>> for i in range (10,0,-1):
... print(i)
...
10
9
8
7
6
5
4
3
2
1
The value to start with is 10
. As we discussed earlier, the
end value is exclusive. So to print from 10
to
1
, we want to end one value which is 0
.
range(10, 0)
seems to be what we need.
Usually these step value is positive, but we need to go backwards from
10
. Hence, we would give a step value of -1
.
Now, let’s print the first 10
even numbers in reverse.
>>> for i in range (20,0,-2):
... print(i)
...
20
18
16
14
12
10
8
6
4
2
Next, we would want to print the squares of the first 10 numbers.
>>> for i in range (1,11):
... print(i * i)
...
1
4
9
16
25
36
49
64
81
100
Let’s print the squares in the reverse order.
>>> for i in range (10,0,-1):
... print(i*i)
...
100
81
64
49
36
25
16
9
4
1
Print the squares of the even numbers. How to do that?
>>> for i in range (10,0,-2):
... print(i*i)
...
100
64
36
16
4
The key part is using a step of -2
We leave it as an exercise for you, to print squares of odd numbers.
In this video, we: * Tried out a few exercises involving the for loop, by playing around with printing sequences of numbers.
It must have been a roller-coaster ride to solve the multiplication table challenge so far. If you’re new to programming, there are a wide range of topics and concepts, that you would have learned during this small journey.
Let’s quickly revise the important concepts we have learned during this small journey.
1
, 11
, 5
, … are all called
literals because these are constant values. Their values don’t really
change. _Consider
5 _ 4 _ 50`. This is an expression. `_`is an operator, and`5`,
`4`and`50
are operands.
i
in i = 1
, is called a variable. It
can refer to different values, at different points in time.
range()
and print()
are in-built Python
functions.
print()
, is invoking a method. The other statement which we
looked at earlier, was an assignment statement.
index = index + 1
would evaluate index + 1
,
and have the index
variable refer to that value.
for
loop was very simple.
for var in range(1, 10) : ...
, followed by statements you
would want to execute in a loop, with indentation. For the sake of
indentation we left four <SPACE>
s in front of each
statement inside the for
loop.
So that, in a nutshell, is what we have learned over the course of our first section.
In the last section, we introduced you to the basics of python. We learned those concepts by applying them to solve the PMT-Challenge problem. The code below is what we ended up with as we solved that chellenge.
>>> for i in range (1,11):
... print(f"8 * {i} = {8 * i}")
If we wanted to change the code to print the 7
table, we need
to change the value 7
used in the for loop, to
8
. It’s simple, but still not as friendly as you would like.
>>> for i in range (1,11):
... print(f"7 * {i} = {7 * i}")
To print a 7
table, it would be awesome if could say
print_multiplication_table
, and give a value of 7 beside it,
and it would do the rest:
>>> print_multiplication_table(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'print_multiplication_table' is not defined
>>> print_multiplication_table(8)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'print_multiplication_table' is not defined
Similarly, print_multiplication_table(8)
, could print the
multiplication table for 8
!
To be able to do this, we need to create a method, or a function. Creating a method makes the code reusable, and we can invoke that method very easily by passing arguments.
In this section, we take an in-depth look at methods.
Methods are very important building blocks in Python programming. In this
step, we will create a simple method that prints
"Hello World"
, twice.
When we talk about a method, we need to give it a name. We are already
using an in-built Python method here, which is print()
.
>>> print("Hello World")
Hello World
>>> print("Hello World")
Hello World
Similar to that, we need to give a name to our body of code. Let’s say the
name is print_hello_world_twice
.
The syntax to create a method in Python is straightforward:
def
followed by a space.
print_hello_world_twice
.
()
.
This is followed by a colon :
(similar to what we used in
a for
loop).
>>> def print_hello_world_twice():
... print("Hello World")
... print("Hello World")
...
All statements in a method should be indented. The two
print("Hello World")
are indented. So, they are part of the
method body.
print_hello_world_twice()
defines a method, and it has
certain code inside its body.
How do we call this method? Is it sufficient to say
print_hello_world_twice
?
>>> print_hello_world_twice
<function print_hello_world_twice at 0x10a71ef28>
Python Shell says, there’s a function defined with that specific name.
How do we execute a method? Very simple! Add a pair of parentheses to the
name, ()
!
>>> print_hello_world_twice()
Hello World
Hello World
>>> print_hello_world_twice()
Hello World
Hello World
Now, we are able to run the method.
In this step, we:
We will now leave you with two exercises, based on what we have learned about methods so far.
Write a method called print_hello_world_thrice()
. It
should print "Hello World"
thrice to the output. Define
this method, and also invoke it.
>>> def print_hello_world_thrice():
... print("Hello World")
... print("Hello World")
... print("Hello World")
...
>>> print_hello_world_thrice()
Hello World
Hello World
Hello World
>>> def print_your_progress():
... print("Statement 1")
... print("Statement 2")
... print("Statement 3")
... print("Statement 4")
...
>>> print_your_progress()
Statement 1
Statement 2
Statement 3
Statement 4
def print_your_progress():
print("Statement 1")
print("Statement 2")
print("Statement 3")
print("Statement 4")
For convenience, we have changed the exact text we need to print. Call
this method with the syntax print_your_progress()
, and you’re
able to execute its code.
Now try another exercise. We want to print "Statement 1"
,
"Statement 2"
, "Statement 3"
and
"Statement 4"
on different lines, using just one print
statement. How can you do that?
>>> def print_your_progress():
... print("Statement 1\nStatement 2\nStatement 3\nStatement 4")
...
>>> print_your_progress()
Statement 1
Statement 2
Statement 3
Statement 4
We are using the newline character \n
.
Let’s look at the difference between defining and executing a method.
When we are writing a method definition, we are writing the code as part
of its body. It has a specific syntax, and starts with the
def
keyword.
A definition by itself cannot cause the code in its body to be executed.
print_your_progress()
represents a method call. The code
inside the method is executed.
In this step, we:
In the previous step,we created methods. We defined
print_hello_world_twice()
, and this printed
"Hello World"
twice. In this step, let’s talk about
method arguments, or parameters.
>>> print_hello_world_twice()
Hello World
Hello World
>>> print_hello_world_thrice()
Hello World
Hello World
Hello World
Earlier, we wrote code for print_hello_world_thrice()
, which
prints the message three times.
Let’s say you want to print it five times. You would need to write another method that does what you need. Doesn’t that seem monotonous?
Instead of that, Won’t it be great if I can call the method by the same
name, say print_hello_world(5)
, and it would print “Hello
World” five times?
The 5
which we are passing here is called an
argument.
How do we define our method to accept this argument?
Let’s call our argument no_of_times
. If you have any
experience with other programming languages, they generally need you to
specify the parameter type. Something like
This parameter is an integer/float/string, or other types
.
But Python does not require parameter type.
>>> def print_hello_world(no_of_times):
... print("Hello World")
... print(no_of_times)
...
Although we are not doing exactly what we set out to, let’s see what would
happen. What would happen if we say print_hello_world()
?
>>> print_hello_world()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: print_hello_world() missing 1 required positional argument: 'no_of_times'
Error! Something like “Hey, you have created
print_hello_world
with a parameter, but not passing anything
in here! Go ahead and pass a value”. Let’s pass in a value, such as
5
.
>>> print_hello_world(5)
Hello World
5
>>> print_hello_world(10)
Hello World
10
>>> print_hello_world(100)
Hello World
100
With print_hello_world(5)
, you can see
"Hello World"
and 5
being printed. We are now
able to define this method to accept a value, and print that value by
invoking it. You can pass in any value, such as10
,
100
, or others.
Now think of a different solution for this method, where you don’t repeat
the same piece of code to print "Hello World"
. Consider
print_hello_world(5)
, it should still print
"Hello World"
5
times. How do you do that?
Think about using something along the lines of a loop.
For now, what we are doing is we are printing "Hello World"
10
times.
>>> def print_hello_world(no_of_times):
... for i in range(1,10):
... print("Hello World")
...
>>> print_hello_world(5)
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Our method call print_hello_world(5)
now prints
"Hello World"
10
times.
However just print the message 5
times. We need to make use
of the parameter no_of_times
inside the for
loop
as well.
>>> def print_hello_world(no_of_times):
... for i in range(1,no_of_times):
... print("Hello World")
...
>>> print_hello_world(5)
Hello World
Hello World
Hello World
Hello World
Now let’s execute the method again. You can see that it’s printing
4
times only.
Why is it not printing 5
times?
That’s because no_of_times
as a second parameter to
range()
is exclusive.
>>> def print_hello_world(no_of_times):
... for i in range(1,no_of_times+1):
... print("Hello World")
...
>>> print_hello_world(5)
Hello World
Hello World
Hello World
Hello World
Hello World
Great, it’s now printing the message 5
times!
>>> print_hello_world(7)
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
If you pass a different argument like 7
, the message is
displayed 7
times.
Something you need to always be cautious about in Python, is the
indentation. Over here, the for
loop is part of the method
body. So we have extra indentation for it. The print is part of the
for
loop body. So guess what, even more indentation for that
code.
In this step, we:
In this step, Let’s look at a few exercises related to the method parameter.
Write a method called print_numbers()
, that would print
all successive integers from 1
to n
.
The second one is to write a method called
print_squares_of_numbers()
, that prints squares of all
successive integers from 1
to n
.
>>> def print_numbers(n):
... for i in range(1, n+1):
... print(i)
...
>>> print_numbers(5)
1
2
3
4
5
>>>
If you are programming in other languages such as Java, you are used to
naming methods in this way: printNumbers()
. This convention
is popularly known as “Camel Case”.
That’s NOT how Python programmers name their methods. Pythonic way is to
use underscore _
to separate words in the method name, as in
print_numbers()
.
Let’s define print_squares_of_numbers()
. This would be very
similar to print_numbers()
, working with the same range.
Only, we need to say print(i*i)
.
>>> def print_squares_of_numbers(n):
... for i in range(1, n+1):
... print(i*i)
...
>>> print_squares_of_numbers(5)
1
4
9
16
25
How is a parameter different from an argument?
n
is a parameter, because it’s used in the definition of
print_squares_of_numbers
.
5
, that value is called an argument.
In this step, we looked at a few simple exercises related to passing method arguments
In this step, let’s look at creating a method with multiple parameters.
print_hello_world
accepts one parameter and prints “Hello
World” the specified number of times.
>>> def print_hello_world(no_of_times):
... for i in range(1,no_of_times+1):
... print("Hello World")
...
Let’s say we want to print another piece of text
Welcome To Python
, a specified number of times. How do you do
that?
You can always create another method similar to the first one, such as
print_welcome_to_python(no_of_times)
and print the necessary
text inside.
However, is that what a good programmer does?
A good programmer tries to create a more generic solution.
>>> def print_string(str, no_of_times):
... for i in range(1,no_of_times+1):
... print(str)
...
>>> print_string("Hello World", 3)
Hello World
Hello World
Hello World
The good programmer that you are, you created a new method called
print_string(str, no_of_times)
accepting a text parameter, in
addition to no_of_times
.
Syntax rules for method parameters are quite strict. If we say
print_string("Welcome to Python")
and run it, we get an
error! Python Shell says: “I need no_of_times
to be present
in here”.
>>> print_string("Welcome to Python")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: print_string() missing 1 required positional argument: 'no_of_times'
Let’s say you want to assign default values for str
and
no_of_times
in print_string()
. By default, we
want to always print "Hello World"
, and that too
5
times.
The Python language makes this very easy.
def print_string(str = "Hello World", no_of_times=5)
. The
rest of the method remains the same.
>>> def print_string(str="Hello World", no_of_times=5):
... for i in range(1,no_of_times+1):
... print(str)
...
Now you can call print_string()
, and
"Hello World"
is displayed 5
times.
>>> print_string()
Hello World
Hello World
Hello World
Hello World
Hello World
If it’s print_string("Welcome To Python")
, what does it do?

In this step, we:
Let’s get back to our original goal, of why we needed methods. We wanted to create a multiplication table for a number, and observed that each time we needed to we needed change that number, we were forced to make a change in the code. This is not something we liked, and that’s why we started investigating how methods can be used.
In this step, Let’s try our hand at creating a multiplication table method.
>>> for i in range (1,11):
... print(f"7 * {i} = {7 * i}")
Let’s define a method called print_multiplication_table()
,
and pass in a parameter to it.
>>> def print_multiplication_table(table):
... for i in range(1,11):
... print(f"{table} * {i} = {table * i}")
...
>>> print_multiplication_table(7)
7 * 1 = 7
7 * 2 = 14
7 * 3 = 21
7 * 4 = 28
7 * 5 = 35
7 * 6 = 42
7 * 7 = 49
7 * 8 = 56
7 * 9 = 63
7 * 10 = 70
Now you have the entire multiplication table for 7
.
You can then call print_multiplication_table()
with arguments
8
, 9
,and so on, by simply changing the
table
arguemnt value.
We now want to create even better
print_multiplication_table()
method.
We want to control the start point, as well as the end point, in the call
to range()
. We want to say
print_multiplication_table(7, 1, 6)
, to print the
7
table with entries from 1
to 6
.
How can you do that?
>>> def print_multiplication_table(table, start, end):
... for i in range(start, end+1):
... print(f"{table} * {i} = {table * i}")
...
>>> print_multiplication_table(7, 1 , 6)
7 * 1 = 7
7 * 2 = 14
7 * 3 = 21
7 * 4 = 28
7 * 5 = 35
7 * 6 = 42
Simple! Define those range limits as additional parameters!
The other thing we can obviously do, is have default values for the
start
, and the end
.
>>> def print_multiplication_table(table, start=1, end=10):
... for i in range(start, end+1):
... print(f"{table} * {i} = {table * i}")
...
>>> print_multiplication_table(7)
7 * 1 = 7
7 * 2 = 14
7 * 3 = 21
7 * 4 = 28
7 * 5 = 35
7 * 6 = 42
7 * 7 = 49
7 * 8 = 56
7 * 9 = 63
7 * 10 = 70
Calling print_multiplication_table(7)
would give us entries
from 7 * 1 = 7
to 7 * 10 = 70
.
Now you can actually send out this method, to your friends, who would find it easy to use, and cool!
In this step, we:
In Python, indentation denote blocks of code. So if you want to put
something in a for
loop, or outside it, proper indentation
would be sufficient. In this step, let’s explore indentation in depth.
Let’s start by creating a simple method.
>>> def method_to_understand_indentation():
... for i in range(1,11) :
... print(i)
...
>>> method_to_understand_indentation()
1
2
3
4
5
6
7
8
9
10
Consider the code below: print(5)
is indented at the same
level as for loop
.
>>> def method_to_understand_indentation():
... for i in range(1,11) :
... print(i)
... print(5)
...
You can see that print(5)
is called only once. It is not part
of the for loop
.
>>> method_to_understand_indentation()
1
2
3
4
5
6
7
8
9
10
5
Let’s change the code in this method a bit. print(5)
is
indented the same way as print(i)
>>> def method_to_understand_indentation():
... for i in range(1,11) :
... print(i)
... print(5)
...
print(5)
is part of the for loop. It is executed 10 times.
>>> method_to_understand_indentation()
1
5
2
5
3
5
4
5
5
5
6
5
7
5
8
5
9
5
10
5
Whether we’re talking about loops, methods or conditionals, proper indentation is very important in Python.
We indicate a block of code, by having all lines of that block at the same
indentation level. There are no specific delimiters like for instance a
pair of braces {...}
, as in other programming languages.
In this step, we:
In this step, let’s look at a variety of puzzles related to methods.
Consider the following method: I would want to print the default string 6 times. How do we do it?
>>> def print_string(str="Hello World", no_of_times=5):
... for i in range(1,no_of_times+1):
... print(str)
...
>>> print_string()
Hello World
Hello World
Hello World
Hello World
Hello World
Will it work if we call the method as in: print_string(6)
?
>>> print_string(6)
6
6
6
6
6
6
is passed as the first parameter. 6
is matched
to str
, and the method prints 6
the default
number of times, which is 5
.
to default to "Hello World"
, and print it
6
times.
You can do this in Python by using named parameters.
During the method call, you can specify no_of_times = 6
.
no_of_times
is a named parameter.
There is no provision of doing something like this, in other languages like Java.
Call it as print_string(no_of_times=6)
:
>>> print_string(no_of_times=6)
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
str
gets a default value, and "Hello World"
is
printed 6
times.
Named parameters are very useful, when a method has a number of parameters, and you would want to make it very clear which parameter you’re passing a value for.
Let’s call print_string(7, 8)
. what happens?
>>> print_string(7, 8)
7
7
7
7
7
7
7
7
You would see that 7
is printed 8
times.
Since print()
method is quite flexible, you can pass a number
as the first argument. You can even pass a float
.
>>> print_string(7.5, 8)
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
What would be the result of this -
print_string(7.5, "eight")
?
>>> print_string(7.5, "eight")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in print_string
TypeError: must be str, not int
Note how no_of_times
is used inside the method… as an
argument to range()
. range()
only accepts
integers, nothing else. When you run the code with
print_string(7.5, "eight")
, we get an error.
It says:
TypeError: ```no_of_times``` must be ```int```, not string
.
A simple rule of thumb is, if you have a parameter, you can pass any type
of data to it. That could be an integer, a floating point value a string,
or a boolean value. The Python language does not check for the type of a
parameter. However, Python will throw an error if the function which is
using that parameter, expects it to be of a specific type. The
range()
function expects that the no_of_times
is
an integer value.
The last thing which we would be looking at, is method naming conventions.
We named our methods in a consistent way: print_string
,
print_multiplication_table
, and the like.
This is exactly the format which most Python developers use, to name their methods.
Convention is to use underscore to separate words in a name.
However, there are a few rules for naming a method: One of the important rules is also related to variable names. We observed that a variable name cannot start with a number.
>>> def 1_print():
File "<stdin>", line 1
def 1_print():
^
SyntaxError: invalid token
Similarly, 1_print
will not be accepted as a method name.
Methods and variables cannot be named using Python keywords.
Now, what is a keyword? For example, when we talked about
for
loop, as in:
```for i in range(1, 11): print(i)```...
for
is a keyword
in
is a keyword
def
is a keyword.
Later we will look at a few other keywords, such as
while
, return
, if
, else
, elif
, and many more.
>>> def def():
File "<stdin>", line 1
def def():
^
SyntaxError: invalid syntax
>>> def in():
File "<stdin>", line 1
def in():
^
SyntaxError: invalid syntax
>>> def for():
File "<stdin>", line 1
def for():
^
SyntaxError: invalid syntax
In this step, we:
Let’s try and understand the importance of return values from a method. We will learn how to return a value from a method.
Let’s name our method as product_of_two_numbers()
, and let’s
have parameters a
and b
that it accepts:
>>> def product_of_two_numbers(a,b):
... print(a * b)
...
>>> product_of_two_numbers(1,2)
2
Can we take the product of these two numbers into a variable, and use it in other code, in the same program?
Suppose we say a product = product_of_two_numbers(1,2)
, is
this allowed?
Let’s run this code, and see what’s stored in product
.
>>> product = product_of_two_numbers(1,2)
2
>>> product
It’s empty.
The product_of_two_numbers()
method is not really returning
anything back, to be used elsewhere.
Have a look at some of the built-in Python functions, such as
max()
for example.
>>> max(1,2,3)
3
>>> max(1,2,3,4)
4
>>> maximum = max(1,2,3,4)
>>> maximum
4
>>> maximum * 5
20
If I call max()
with four parameters, as in
maximum = max(1,2,3,4)
, the value 4
gets stored
in maximum.
Later on in the code that follows, we can say maximum * 5
, or
we can print the value of maximum
, or a similar calculation.
This gives our programs a lot more flexibility.
So instead of just printing a*b
, if this function could
return a value, that would be quite useful.
>>> def product_of_two_numbers(a,b):
... product = a * b;
... return product
...
>>> product_of_two_numbers(2,3)
6
We are creating a variable product
and doing a
return product
.
Lets run product_result = product_of_two_numbers(2, 3)
>>> product_result = product_of_two_numbers(2,3)
>>> product_result
6
>>> product_result * 10
60
You can see how simple it is to return values from a method!
In this step, we:
In this step let’s look at a couple of exercises about returning values from methods.
Write a method to return the sum of three integers.
Write a method which takes as input two integers, representing two angles of a triangle, and computes the third angle.
Hint: The sum of the angles in a triangle is 180
degrees. So
if I am passing 50
and 50
, 50
plus
50
is 100
. So some of three angles should be
180
, so the third angle will be 180 - 100
, which
is 80
.
>>>def sum_of_three_numbers(a, b, c):
... sum = a + b + c
... return sum
...
>>> sum_of_three_numbers(1,2,3)
6
>>> something = sum_of_three_numbers(1,2,3)
>>> something * 5
30
The shorter way of doing that would have been to have a temporary variable
called instead of sum
. We could directly
return a + b + c
.
>>> def sum_of_three_numbers(a, b, c):
... return a + b + c
...
>>> something = sum_of_three_numbers(1,2,3)
>>> something * 5
30
In methods, you can use return expression
as well. That
expression
gets evaluated, and the value gets returned back.
You’d see that the result remains the same.
The second is to write a method to take two integers, representing two angles of a triangle, and compute the third one.
>>> def calculate_third_angle(first, second) :
... return 180 - ( first + second )
...
>>> calculate_third_angle(50, 20)
110
In your programming career, you would be writing a number of methods. It’s very important that you are comfortable doing so. Most of the methods that you write would return values back.
That’s the reason why we’re creating a lot of examples involving method calls.
In this step, we:
Until now we had been using Python Shell to execute all our code.
In the real world, we’ll be write Python code in a variety of scripts. Before we would go into an IDE and use the IDE to write the script, we thought it would be useful for us to understand how you can write Python code without the benefit of an IDE.
This would also help us understand the Python environment, in-depth.
In the next few steps, we’ll be looking at how to create simple Python scripts, using any text editor of your choice. Use Notepad, Notepad++. Editpad, or whichever text editing software you are comfortable with. We’ll see what involved in executing the program, and what’s happening in the background.
Here are a few videos you might want to look at.
Here’s a recommended video to watch - Writing and Executing your First Python Script
Let’s get started with creating a simple script file.
We want to type in a simple Python script, or a piece of Python code, such
as print("Hello world")
. Does it get any simpler than this?
We’ll save this into any folder on our hard disk, with a name ‘first.py’ .
first.py
print("Hello world")
The ‘.py’ is not really mandatory, but typically all python files end with a ‘.py’ extension.
Here’s how you can run it:
python first.py
You will see that Hello World
will be printed.
If you are familiar with other programming languages, you would need a class, need to put the code in that class, and similar stuff.
While Python supports Object Oriented Programming, is not mandatory to create a class.
It’s almost as if you’re typing commands, starting from the line one! That’s why we call it a python script.
In this small step, we tried to create a simple python script, and we ran it from the command line. All we needed to do, was use the same command we use to launch up the python shell, and followed it up with a name of the file. We created a file called first.py, executed that, and were able to see the output on the console.
As an exercise, try and add a few more methods and try to run those methods as well, as part of this script.
In this step, let’s try and understand what’s happening in the background.
We wrote a simple piece of code using a text editor. We created a file
named first.py, and all we did was: python3 first.py
. If you
look at other languages like Java for example, there is a separate
compilation phase and then an execution phase. But with Python, just this
command does both compilation and execution.
We saw that, as soon as we make a change and we run
python3 first.py
, the change is compiled and executed as
well!
In Python, there is an intermediate format called Python byte code. Code is first compiled to bytecode, and then executed on the Python virtual machine.
When we installed Python, we installed both the python compiler and interpreter, as well as the virtual machine.
In Python, bytecode
is not standardized. Different
implementations of Python have different byte code. There are about 80
Python implementations, like CPython and Jython.
Python leaves a lot of flexibility to the implementations of Python. They have the flexibility to choose the bytecode, and to choose the virtual machine that is compatible. The bytecode is tied to the specific virtual machine you are using. Therefore, if you’re using CPython to compile the bytecode, you’ll not be able to use Jython to run it.
You should make sure, that whatever implementation you are using to compile, is the same one you’re using to run the code as well.
A lot of this sounds like boring theory. Don’t worry about it. As a beginner, this might not be very important for you right now.
It’s very important for you to understand the process. What’s happening is
you were writing Python code, and when you ran the command
python3 first.py
, it is both compiled and executed. An
intermediate format called bytecode is created, which is not really
standardized in Python. The bytecode is executed in a Python virtual
machine.
The idea behind this quick section, is to give you a little bit of background on what’s happening behind the scenes. I’ll see you in the next section. Until then, bye-bye!
Let’s start using the IDE VSCode to write our Python Code
Here are recommended videos to watch
In this quick step, we’ll help you install VSCode.
Here’s the video guide for this step
Go to Google and type in “VSCode Community Edition Download”. Click the link which comes up first: https://www.jetbrains.com/VSCode/download.
You’ll go to a page where you can choose the operating system: whether you are on Windows, Mac, or Linux.
Once you choose that, you can download the appropriate community version.
On the right hand side, you’ll see a community version, and you can click the download link, to start the download.
If you are having a problem, you can also use the direct link to download.
Once you download VSCode, all you need to do is double-click the package which is downloaded. Follow the instructions, and you can continue with the defaults, until you completely install VSCode.
When you launch VSCode for the first time, it should ask you for a theme, where you can choose the default.
You’re all set to go ahead with the next step in the course.
VSCode is an awesome IDE, and I’m sure you learn a lot about it.
In this step, let’s launch up the VSCode IDE, and create our first Python project with a Python script. We want to be able to launch a Python script by the end of this step.
Here’s the video guide for this step
Launch the VSCode IDE. You’ll see that it takes a little while to launch the first time, and then brings up a welcome screen.
We would want to create a number of Python files. All these files will be in a project. You can think of our project as a collection of Python scripts, or modules.
To get started, let’s create a new project by clicking ‘create new project’. Let’s name it - ‘01-first-python-project’.
Right now there are no files in the project.
Let’s create our first Python file, using the IDE.
The way you can do that is by saying ‘right-click’ -> ‘new’ -> ‘Python file’, and then we’ll give this a name of ‘hello_world’, and click OK.
Now you can go ahead and write your first Python program. Let’s write some
simple code, like print("Hello World")
, and save it.
You can do a right-click here, and say ‘Run hello_world’.
A small window comes up below, which shows the output. It says
'Hello World'
.
Let’s start with a simple exercise. We created the multiplication table method in the Python Shell. What we do now, is we’ll create the same thing but in a Python file of its own.
Here’s the video guide for this step:
Welcome to this section, where we will talk about numeric data types, and conditional program execution. After looking at the numeric and boolean data types, we will turn our attention to executing code, based on logical conditions.
In previous sections, we created variables of this kind:
number = 5
, value = 2.5
, etc. The
5
here is an integer, and integers represent numbers, such as
1
, 2
, 6
, -1
and
-2
. In Python, the class
for this particular
data type is int
.
If you write code like type(5)
, you’d get
'int'
as the output.
In Python, there are no primitive types. What does that mean? Every value
that you see in a Python program, is an object, an instance of some
class
.
In later sections, We’ll understand what is a class
, and what
is an object or an instance. For now, the most important thing for you to
remember, is that behind every value, there is a class
.
Let’s look at 2.5
, which is a floating point value.
If you go ahead and do type(2.5)
, what would you see? You
would see it’s of type `float
.
>>> type(2.5)
<class 'float'>
>>> type(2.55)
<class 'float'>
When you perform a division operation between two integers, there is a
chance that the result of the operation is a float
. If you do
5/2
, the result is 2.5
. If we were to do
4/2
, even then it’s of type float
.
>>> type(5/2)
<class 'float'>
>>> type(4/2)
<class 'float'>
>>> 4/2
2.0
>>> 1 + 2
3
All the operations we looked at until now, can also be performed on floating point values.
>>> value1 = 4.5
>>> value2 = 3.2
>>> value1 + value2
7.7
>>> value1 - value2
1.2999999999999998
>>> value1 / value2
1.40625
>>> value1 % value2
1.2999999999999998
value1 - value2
returns 1.299999999999998
. Why?
Floating point numbers don’t really represent accurate values. That’s one of the things you need to always keep in mind.
Typically, if you’re doing any highly sensitive financial calculations,
don’t use float
s to represent your values. Instead, use
Decimal
. More about it later.
Operations can also be performed between int
and
float
.
>>> i + value1
14.5
>>> i - value1
5.5
>>> i / value1
2.2222222222222223
>>>
Result of an operation between a int
and a
float
, is always a float
.
In this step, we:
int
and
float
.
int
s, among
float
s, and also between int
s and
float
s.
In this step, let’s do a simple exercise with numeric values.
simple_interest
, and
pass three parameters: principal
, interest
and
duration
(in years). You also want to calculate the amount
after the specific duration, and return it back. Call this method with a
few example values.
For example, if you want to call simple_interest
with
10000
, with an interest of 5
percent, for a
duration of 5
years, the correct answer would be as follows:
10000
is the principal. In addition to 10000
,
you get the interest. The interest for one year is
10000 * 0.05
, as the interest figure is in percentage.So
that’s 500
a year, into 5
which is
2500
. The result would be 12500
, and this value
should be printed.
def calculate_simple_interest(principal, interest, duration) :
return principal * (1 + interest * 0.01 * duration)
print(calculate_simple_interest(10000,5,5))
In this step, we:
In this section, we are looking at numeric types. In this specific step, we would be looking at a few puzzles related to values of these types.
Let’s create a simple variable i = 1
. i = i + 1
.
What would be the value of i
after that?
>>> i = 1
>>> i = i + 1
>>> i
2
It would be 2
. There is a shortcut way of doing the same
thing, by using the +=
operator.
>>> i += 1
>>> i
3
Typically in other programming languages, you can do something of this
kind: i++
. There is no provision in Python to use increment
operators like ++
, in either prefix or suffix mode, like
++i
, or i++
.
>>> i++
File "<stdin>", line 1
i++
^
SyntaxError: invalid syntax
>>> ++i
3
Let’s look at compound assignments.
>>> i += 1
>>> i
4
>>> i -= 1
>>> i
3
>>> i /= 1
>>> i *= 2
>>> i
6.0
What you see here, is Dynamic Typing in Python. The type of a variable can change during the lifetime of the program.
>>> i = 2
>>> type(i)
<type 'int'>
>>> i = i/2.0
>>> type(i)
<type 'float'>
Let’s create a couple more numbers. number1 = 5
and
number2 = 2
. What could be the result of
number1 / number2
? You know it, it’s 2.5
.
number1 // nummber2
truncates the value of 2.5
,
to 2
.
>>> number1//number2
2
If you can do number1 // number2
, can you also do this:
number1 //= number2
?
>>> number1 //= 2
>>> number1
2
5 ** 3
is 5
‘to the power of’ 3
,
which is 5 * 5 * 5
, or 125
.
>>> 5 ** 3
125
>>> pow(5,3)
125
This can also be achieved by invoking pow(5, 3)
. We have an
operator, as well as a method at our disposal.
The last thing we will look at, are type conversion functions.
If you need to convert an int
value to a float
,
or a float
to an int
.
>>> int(5.6)
5
What if you want to round a value? 5.6
is nearer to
6
than 5
. You can use a function called
round()
, and here,round(5.6)
gives the correct
result 6
.
>>> round(5.6)
6
>>> round(5.4)
5
>>> round(5.5)
6
round()
can also allows you to specify number of decimals in
the result.
>>> round(5.67, 1)
5.7
>>> round(5.678, 2)
5.68
You can also convert int
to float
, by using the
function float()
.
>>> float(5)
5.0
In this step, we:
We will now shift our attention to the bool
data type.
A boolean value is something which can be either “true” or “false”.
In Python, “true” is represented by True
, and “false” by
False
. It’s important to remember that it’s
True
with a capital 'T'
, and
False
with a capital 'F'
.
>>> True
True
>>> False
False
>>> true
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'true' is not defined
>>> false
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'false' is not defined
The boolean variable is_even
indicates whether a number is
even or not.
>>> is_even = True
>>> is_odd = False
Let’s create a variable i = 10
. We want to find out if
i > 15
. What do you think is the result?
False
.
>>> i = 10
>>> i > 15
False
>>> i < 15
True
In general, boolean values can represent the result of logical conditions.
Let’s look at other operations that can result in
bool
values. We looked at >
and
<
. Another operation which you can perform, is
>=
.
>>> i >= 15
False
>>> i >= 10
True
>>> i > 10
False
>>> i <= 10
True
>>> i < 10
False
==
is the comparison operator. We are only comparing the
value of i
against 10
, not changing its value.
>>> i == 10
True
>>> i == 11
False
In this step, we:
bool
data typebool
variables are useful handy while testing
logical conditions
In this step, let’s look at if
statement.
Sometimes you need to execute code only when certain conditions are true.
You can use a if
condition, which is the simplest conditional
in Python. Let’s look at an example.
Let’s say i
has a value of 5
. You want to print
something, only if i
has a value greater than 3
.
How do you do that?
>>> i = 5
>>> if i>3:
... print(f"{i} is greater than 3")
...
5 is greater than 3
The syntax of the if
is very simple: if
followed
by a condition; with the condition you want to check. It looks like:
if i>3: ...
You need to indent the body of the
if
with <SPACE>
s as usual.
Let’s say i
has a value of 2
. What would happen
if we execute the same code again?
>>> i = 2
>>> if i>3:
... print(f"{i} is greater than 3")
...
You would see that nothing is printed to the console. Based on the value
of i
, either the statement is executed, or it’s not. That’s
what an if
helps us to do.
The way you can think about an if
, is the body of code under
the if
is executed only when this condition is
True
. If this condition is not True
, that code
is not executed at all.
>>> if(False):
... print("False")
...
>>> if(True):
... print("True")
...
True
Let’s take two different numbers, say a = 5
, and
b = 7
. We want to compare them, and predict if
a
is greater that b
.
>>> a = 5
>>> b = 7
>>> if(a>b):
... print("a is greater than b")
...
>>> a = 9
>>> if(a>b):
... print("a is greater than b")
...
a is greater than b
In this step, we:
if
statement, the simplest Python
conditional
if
helps in implementing conditional
program logic
In this step, let’s look at a couple of exercises with the if statement.
Let’s say we define four variables: a = 1
,
b = 2
, c = 3
and d = 5
. we want to
find out, if a + b
is greater than c + d
.
>>> a = 1
>>> b = 2
>>> c = 3
>>> d = 5
>>> if a+b > c+d :
... print("a+b > c +d")
...
>>> a = 9
>>> if a+b > c+d :
... print("a+b > c +d")
...
a+b > c +d
Let’s say we are given three values meant to be the angles of a triangle.
Their values are angle1 = 30
, angle2 = 20
and
angle3 = 60
. You want to find out if these three angles
actually form a valid triangle. You know that the sum of the angles of a
triangle is always 180
degrees.
>>> angle1 = 30
>>> angle2 = 20
>>> angle3 = 60
>>> if(angle1 + angle2 + angle3 == 180):
... print("Valid Triangle")
...
>>> angle2 = 90
>>> if(angle1 + angle2 + angle3 == 180):
... print("Valid Triangle")
...
Valid Triangle
The last exercise is to check if a number is even or not.
Hint L you need to use one of the operators we talked about earlier.
That’s right, use the modulo operator %
.
>>> i = 2
>>> if(i%2==0):
... print("i is even")
...
i is even
>>> i = 3
>>> if(i%2==0):
... print("i is even")
...
In this step, we:
In this step, let’s look at the different operators that can be used on
bool
values. These operators are called logical operators -
and
, or
, not
and
^
(xor).
Let’s say we have a value True
, and the other
False
, and we want to play around with them.
Logical operator and
returns true only when both operands are
True
.
>>> True and False
False
>>> True and True
True
>>> True and False
False
>>> False and True
False
>>> False and False
Logical operator or
returns true when atleast one of the
operands is True
.
False
>>> True or False
True
>>> False or True
True
>>> True or True
True
>>> False or False
Logical operator not
returns negation.
False
>>> not True
False
>>> not(True)
False
>>> not False
True
>>> not(False)
True
The XOR operation, denoted by the ^
operator, is
True
when operands have different boolean values.
>>> True ^ True
False
>>> True ^ False
True
>>> False ^ True
True
>>> False ^ False
False
In this step, we:
and
, or
, not
and ^
True
, and when False
.
In this step, Let’s look at a few simple puzzles to look at the logical operators.
Let’s say i
has a value of 10
, and
j
has a value of 15
. You want to find out if
both i
and j
are even. How do you do that?
>>> i = 10
>>> j = 15
>>> if i%2==0 and j%2==0:
... print("i and j are even")
...
>>> j = 14
>>> if i%2==0 and j%2==0:
... print("i and j are even")
...
i and j are even
>>> if i%2==0 or j%2==0:
...
File "<stdin>", line 2
^
IndentationError: expected an indented block
>>> if i%2==0 or j%2==0:
... print("atleast one of i and j are even")
...
atleast one of i and j are even
If we want to find out if at least one of i
and
j
is even, we can use the or
operator.
>>> i = 15
>>> j
14
>>> if i%2==0 or j%2==0:
... print("atleast one of i and j are even")
...
atleast one of i and j are even
>>> j = 23
>>> if i%2==0 or j%2==0:
... print("atleast one of i and j are even")
...
>>> i
15
Now try and guess the value of this.
if(True ^ False): print("Message")
>>> if(True ^ False):
... print("This will Print")
...
This will Print
>>> if(False ^ True):
... print("This will Print")
...
This will Print
>>> if(True ^ True):
... print("This will Print")
...
Xor operation using ^
- message will get printed if the
operands are different.
What would happen if both of them are True
? No message is
printed.
So you would use ^
in situations, where you’d want one of the
operands to be True
, and the other to be False
.
Let’s say, x = 5
, and you want to check
if not x == 6: print("This")
. What will be the result of
running this code?
>>> x = 5
>>> if not x == 6:
... print("This")
...
This
>>> x = 6
>>> if not x == 6:
... print("This")
...
Actually, there is a shortcut for such a condition:
if x != 6 : print("This")
.
>>> if x!=6:
... print("This")
...
>>> x=5
>>> if x!=6:
... print("This")
...
This
int()
is a conversion function, which when given say a
float
value, returns an int
value. Consider
int(True)
, what would happen?
>>> int(True)
1
>>> int(False)
0
int(True)
returns 1. int(False)
returns 0.
>>> x = -6
>>> if x:
... print("something")
...
something
One of the most interesting facts about boolean stuff, is anything which
is non-zero, is considered to be True
.
0
is the only integer value which is considered to be
False
.
>>> bool(6)
True
>>> bool(-6)
True
>>> bool(0)
False
>>>
So, if I have a value of x = -6
, and execute
if x: print("something")
what do you think will happen?
"something"
will be printed.
You can use the function bool()
, to convert
int
to a bool
value.
bool(6)
returns True
bool(-6)
returns True
bool(0)
returns False
.
Except for bool(0)
, all the other results would be
True
.
In this step, we:
bool()
and
int()
to convert between boolean and integer data
In this step, let’s look at two other important components of an
if
statement: else
and elif
. Let’s
start with else
.
Consider a scenario where i
has a value of 2
.
Let’s try to print a message "i is even"
if i
is
an even number. Otherwise, print "i is odd"
.
Earlier we wrote code along these lines:
if i % 2 == 0 : print("i is even")
. However if this condition
is not True
, we would want to print("i is odd")
.
How do we accomplish that?
>>> i = 2
>>> if i%2 == 0:
... print("i is even");
... else:
... print("i is odd");
...
i is even
An else
clause provides an alternative code body to execute,
if the if
condition is False
.
>>> i = 3
>>> if i%2 == 0:
... print("i is even");
... else:
... print("i is odd");
...
i is odd
Let’s look at elif
.
We want to do something if i
has value of 3
, and
something totally different if i
has a value of
4
.
In short, we want to specify 2 alternatives to the
if
condition. How can that be done?
>>> if i==1:
... print("i is 1")
... elif i==2:
... print("i is 2")
... else:
... print("i is not 1 or 2")
...
i is not 1 or 2
>>>
That’s where the elif
clause comes into the
picture. The code in elif
is executed if the previous
conditions are false and the current elif
condition is true.
In this step, we:
if
statement:
else
and elif
.
elif
clauses and the final
else
clause provide alternative conditions to check, when
earlier if conditions are true.
In this step, let’s do a simple exercise with if
,
else
and elif
.
Before getting to the exercise, let’s try and learn how to get console input from the user.
Until now, we had been hard-coding all the data we were to use. Let’s make that part more dynamic now.
How do we get input from the user? We want to get input from the console,
and assign it to a variable. The way we can do that, is by statement
value = input()
value = input("Enter a Value: ")
print("you entered ", value)
We can call the input()
method with a text ‘prompt’, such as
"Enter A Value: "
. What we can initially do here, is print
the value which was entered, back to the console, by
print("you entered ", integer_value)
.
An interesting point to explore here, is the type of data input at the console.
Let’s do a print(type(value))
.
value = input("Enter a Value: ")
print("you entered ", value)
print(type(value))
Input a value of Test
. It has a class of str
.
Let’s run it again to see other possibilities. This time, let’s enter a
numeric value, say 12
. what would happen?
We again get str
.
We want to get an integer value from the input. How can we do it?
int()
function converts string to int. Let’s use it.
value = input("Enter a Value: ")
integer_value = int(value)
print("you entered ", integer_value)
print(type(integer_value))
Let’s run our code once again.
"Enter A Value: "
is prompted, and we enter 15
.
And now, of it says "You entered 15"
, and the type it
indicates to us, is int
.
Let’s design a menu, and then ask the user for input.
We have codes for each of the operations : add is 1
, subtract
is 2
, divide is 3
, and multiply is
4
.
In the first version of the program let’s get all the inputs and print them out.
The first version of the program is simple to write
number1 = int(input("Enter Number1: "))
number2 = int(input("Enter Number2: "))
print(f"You entered {number1}")
print(f"You entered {number2}")
print(number1 + number2)
print("\n\n1 - Add")
print("2 - Subtract")
print("3 - Divide")
print("4 - Multiply")
print("5 - Exit")
choice = int(input("Choose Operation: "))
print(choice)
We will continue this exercise to complete it, in the next step.
In this step, we:
input()
function that can read
console input
input()
always returns what the user enters,
as a string
input()
, to the data type we
expect by invoking conversion functions
In the previous step, we got the input from the user. Let’s continue the exercise in this step. We want to write an if condition.
Extending the solution is easy. Write appropriate if
,
elif
and else
conditions.
number1 = int(input("Enter Number1: "))
number2 = int(input("Enter Number2: "))
print("\n\n1 - Add")
print("2 - Subtract")
print("3 - Divide")
print("4 - Multiply")
choice = int(input("Choose Operation: "))
# print(number1 + number2)
# print(choice)
if choice==1:
result = number1 + number2
elif choice==2:
result = number1 - number2
elif choice==3:
result = number1 / number2
elif choice==4:
result = number1 * number2
else:
result = "Invalid Choice"
print(result)
We added the following code to account for invalid input.
else:
result = "Invalid Choice"
In this step, we:
In this step, let’s look at a few puzzles related to these
if
, elif
and else
clauses.
Let’s start with the first puzzle. Guess the output.
k = 15
if (k > 20):
print(1)
elif (k > 10):
print(2)
elif (k < 20):
print(3)
else:
print(4)
When we run it, you can see that the output is 2
.
k
has a value of 15
, is it greater than
20
? No! Execution goes to the elif
, is
k
greater then 10
? Yes. It prints
2
and goes out of the complete if
-else
block.
Inside the if
conditional, the if
,
elif
and else
clauses are all independent ones.
Only one matching block is ever executed.
What do you think would be the output of this particular piece of code?
l = 15
if (l < 20):
print("l<20")
if (l > 20):
print("l>20")
else:
print("Who am I?")
Note that there are two totally different if
conditions in
here : if l < 20: ...
immediately followed byif l > 20: ... else: ...
.
The first if
is true. l<20
is printed.
The second if
is a separate statement. The condition is
false. So. else
gets executed. Therefore,
"who am I"
gets printed.
Let’s run this code.
m = 15
if m>20:
if m<20:
print("m>20")
else:
print("Who am I?")
You can see that nothing is printed.
The most important thing to focus on here, is indentation.
The second if
block is executed only if the first
if
is true.
What would be the output?
number = 5
if number < 0:
number = number + 10
number = number + 5
print(number)
10
is printed.
The most important thing to focus on here, is indentation.
Only number = number + 10
is part of if
block.
It is not executed because the condition is false.
number = number + 5
is not part of if
. So, it
gets executed.
Let’s add a couple of spaces before number = number + 5
.
What would be the output?
number = 5
if number < 0:
number = number + 10
number = number + 5
print(number)
5
is printed.
Both the statements number = number + 10
and
number = number + 5
are part of if
block. They
are not executed because the condition is false.
In this step, we:
if
,
elif
and else
if
statement
Let’s start looking at another important data type in Python, that’s used
to represent strings. Not surprisingly, it is in fact named
str
!
Let’s look at valid string representations.
>>> message = "Hello World"
>>> message = 'Hello World'
>>> message = 'Hello World"
File "<stdin>", line 1
message = 'Hello World" ^
SyntaxError: EOL while scanning string literal
In Python, you can use either ```or
""` to delimit string
values.
type()
method can be used to find type of a variable.
>>> message = "Hello World"
>>> type(message)
<class 'str'>
The str
class
provides a lot of utility methods.
>>> message.upper()
'HELLO WORLD'
>>> message.lower()
'hello world'
>>> message = "hello"
message.capitalize()
does init caps. Only first character is
changed to uppercase.
>>> "hello".capitalize()
'Hello'
>>> 'hello'.capitalize()
'Hello'
You can also run this directly - 'hello'.capitalize()
. Isn’t
that cool!
That’s because each piece of text in python is an object of the
str
class
, and we can directly call methods of
that class
on str
objects.
Now let’s shift our attention to methods, which gives us more information about the specific contents of a string.
To find if a piece of text contains only lower case alphabets.
>>> 'hello'.islower()
True
>>> 'Hello'.islower()
False
If the first letter is in uppercase, then istitle()
will
return a True
value.
>>> 'Hello'.istitle()
True
>>> 'hello'.istitle()
False
To find if a piece of text contains only upper case alphabets.
>>> 'hello'.isupper()
False
>>> 'Hello'.isupper()
False
>>> 'HELLO'.isupper()
True
isdigit()
checks if a string is a numeric value.
>>> '123'.isdigit()
True
>>> 'A23'.isdigit()
False
>>> '2 3'.isdigit()
False
>>> '23'.isdigit()
True
isalpha()
checks if a string only contains alphabets.
>>> '23'.isalpha()
False
>>> '2A'.isalpha()
False
>>> 'ABC'.isalpha()
True
isalnum()
checks if a string only contains alphabets and/or
numerals.
>>> 'ABC123'.isalnum()
True
>>> 'ABC 123'.isalnum()
False
Lastly, we look at things which you can use, to check characters of a string.
endswith
is self explanatory.
>>> 'Hello World'.endswith('World')
True
>>> 'Hello World'.endswith('ld')
True
>>> 'Hello World'.endswith('old')
False
>>> 'Hello World'.endswith('Wo')
False
startswith
is self explanatory as well.
>>> 'Hello World'.startswith('Wo')
False
>>> 'Hello World'.startswith('He')
True
>>> 'Hello World'.startswith('Hell0')
False
>>> 'Hello World'.startswith('Hello')
True
find
method returns if a piece of text is present in another
string. Returns the first match index.
>>> 'Hello World'.find('Hello')
0
>>> 'Hello World'.find('ello')
1
A value of -1
is returned, if you’re searching for something
which is not present in the string.
If you are searching for 'Ello'
with a capital
'E'
,you’ll not be able to find it. Search is case sensitive.
>>> 'Hello World'.find('Ello')
-1
>>> 'Hello World'.find('bello')
-1
>>> 'Hello World'.find('Ello')
-1
We’ll now try and convert values from one type to another, and try and play around with them.
str
converts boolean value to a text value.
>>> str(True)
'True'
All text value except for empty string represent True. So,
bool
returns True for everything except empty string.
>>> bool('True')
True
>>> bool('true')
True
>>> bool('tru')
True
>>> bool('false')
True
>>> bool('False')
True
>>> bool('')
False
Let’s try and convert a few integer values to strings.
>>>str(123)
'123'
>>> str(12345)
'12345'
>>> str(12345.45678)
'12345.45678'
Let’s do the reverse.
>>> int('45')
45
>>> int('45.56')
ValueError: invalid literal for int()
if we do int('45.56')
, you can see that it throws an error.
It says “I cannot convert this to an int
, as
45.56
is an invalid integer”.
You can also pass an additional parameter to int
indicating
the numeric system - 16 for Hexa decimal, 8 for Octal etc. Default is 10 -
Decimal.
>>> int('45abc',16)
285372
>>> int('a',16)
10
>>> int('b',16)
11
>>> int('c',16)
12
>>> int('f',16)
15
>>> int('g',16)
ValueError: invalid literal for int() with base 16: 'g'
You can also convert string to float.
>>> float("34.43")
34.43
>>> float("34.43rer")
ValueError: could not convert string to float: '34.43rer'
In this quick step, we looked at converting different types to strings,
and converting strings to different types. So we looked at
int
, bool
and float
values, and we
looked at how to convert them to string, and how to convert strings back
to these specific types.
In this step, let’s learn an important fact about strings in Python.
String values are immutable.
What does immutability mean, and why do we say strings are immutable?
Let’s create a very simple string: message = 'Hello'
, and
we’re saying message.upper()
. But what does it do? It prints
'HELLO'
, with all characters in uppercase. Well, what would
happen if you do print(message)
? It says
'Hello'
.
>>> message = "Hello"
>>> message.upper()
'HELLO'
>>> message
'Hello'
You would see we tried change the content of message, but it has not changed.
When we execute message.upper()
, a new string is created, and
it is returned back. Original string remained unchanged. This is called
immutability.
Once you define a string in Python, you’ll not be able to change the value of it.
You can use - “OK. I can do something of this kind:
message = message.upper()
”.
What would happen now?
Will the value of message
get changed? It prints
'HELLO'
, with all caps.
Did the value of message
change? Does this prove that strings
are mutable?
The important thing you need to understand about all this stuff, is how objects are stored inside Python.
There are things called variables, and there are things called objects.
When we run message = 'Hello'
str
class with a values
'Hello'
.
message
'Hello'
is stored into message
In Python, your variables are nothing but a name.
If location of 'Hello'
in memory is A
, then the
value stored in message
is A
.
message
is called a reference.
What happens with message = message.upper()
?
A new object is created with value 'HELLO'
at a different
location B
.
A reference ot location B
is stored into
message
variable.
Summary : The original value at location A
has not changed
and cannot be changed for str
variables. Hence ‘str’ objects
are immutable.
Variables are just names referring to a location. They don’t really contain the value. Variables contain a reference to the location that contains the object.
One of the things that surprises people new to Python, is that there is no character data type in Python.
Typically we have text data types in all the languages, don’t we?
'Hello World'
for example, is text data, and we stored it in
message
. This is called a string.
In other languages, you would have something to represent a single
character symbol. For example in Java, you can have a
char
data type, to store a single character ch
,
in which 'h'
is one character. But in Python, there is no
separate data type to store single characters.
For example, let’s see how Python treats the first character of the
following string message
. The way you can access the first
character of a string is by saying message[0]
.
>>> message = "Hello World"
>>> message[0]
'H'
>>> type(message[0])
<class 'str'>
>>> type(message)
<class 'str'>
type(message[0])
and type(message)
print the
same type str
. No difference.
In Python, whether you’re talking about a string, or you’re talking about
a single character symbol, they are all represented by the same
class
, str
.
message[100]
throws an IndexError
.
>>> message[0]
'H'
>>> message[1]
'e'
>>> message[2]
'l'
>>> message[3]
'l'
>>> message[100]
IndexError: string index out of range
It says: “The given index is out of the range of the value of that specific string”.
Let’s say we would want to print all the characters in this string.
The way you could do that, is by saying:
for ch in message: print(ch)
.
In this short step, we looked at the fact that there is no separate character class, or data type in Python. We also looked at how do we loop over a given string, and print all the characters present inside this string.
string
module
In this step, we will introduce you to the string
module
.
If we would want to use anything from a module in Python, you need to
import that specific module
into your program.
>>> import string
If you do a string.
and press , it would show the different
things which are part of the string
module
.
>>> string.
string.Formatter( string.ascii_uppercase string.octdigits
string.Template( string.capwords( string.printable
string.ascii_letters string.digits string.punctuation
string.ascii_lowercase string.hexdigits string.whitespace
Let’s explore some of these.
>>> string.ascii_letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> string.ascii_lowercase
'abcdefghijklmnopqrstuvwxyz'
>>> string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> string.digits
'0123456789'
>>> string.hexdigits
'0123456789abcdefABCDEF'
>>> string.punctuation
'!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~'
>>> string.ascii_letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
You have a set of printable characters, punctuation characters and a lot more.
You can check a text value against any of these
>>> 'a' in string.ascii_letters
True
>>> 'ab' in string.ascii_letters
True
>>> 'abc' in string.ascii_letters
True
in
operation on a string, checks if a given string.
>>> '1' in '13579'
True
>>> '2' in '13579'
False
>>> '4' in '13579'
False
In this step, we explored more exercises involving the
str
module of Python.
str
Module
Let’s start with an Exercise - find if a specific character is a vowel or not.
>>> char = 'a'
>>> vowel_string = 'aeiouAEIOU'
>>> char in vowel_string
True
>>> char = 'b'
>>> char in vowel_string
False
he other thing you can do, is just have the capital vowels, or just the lowercase versions.
>>> vowel_string = 'AEIOU'
>>> char.upper() in vowel_string
False
>>> char = 'a'
>>> char.upper() in vowel_string
True
Now let’s move on to the next one.
We want to find out and print all the capital alphabets, from
A
to Z
.
There was a small clue at the start of the previous step, regarding
importing the string
module
. We did the
string
module
, and we saw that
string
module
contained a number of things.
>>> string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> for char in string.ascii_uppercase:
... print(char)
...
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Try another easy exercise: print all the lower characters. Instead of
string.ascii_uppercase
, you have
string.ascii_lowercase
.
>>> for char in string.ascii_lowercase:
... print(char)
...
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
An even easier exercise, would be to print all the digits.
>>> for char in string.
string.Formatter( string.ascii_uppercase string.octdigits
string.Template( string.capwords( string.printable
string.ascii_letters string.digits string.punctuation
string.ascii_lowercase string.hexdigits string.whitespace
>>> for char in string.digits:
... print(char)
...
0
1
2
3
4
5
6
7
8
9
>>>
The last exercise which we want to leave you with, is to check if something is a consonant.
A consonant is an alphabet which is not a vowel, so any alphabet which is
not in 'aeiou'
is a consonant. The simplest way of doing this
is to say consonant_string = 'bcdfghj...'
and so on. Looks
like a very long solution? There is an easier way out.
>>> vowel_string = 'aeiou'
>>> char = 'b'
>>> char.isalpha() and char.lower() not in vowel_string
True
In the step, let’s look at a few more puzzles and exercises related to
strings. Let’s say we have a simple string, string_example
,
and this is contains an English sentence.
'This is a great thing.'
Let’s try to to print each of the words present in this string, on a separate line.
So we would want to print 'This'
, 'is'
,
'a'
, 'great'
and 'thing'
on
individual lines.
One of the clues we’ll give you is, try and do
string_example. <TAB>
. There are a huge list of
methods, which would come up if you do that.
>>> string_example = "This is a great thing"
>>> string_example.
string_example.capitalize( string_example.join(
string_example.casefold( string_example.ljust(
string_example.center( string_example.lower(
string_example.count( string_example.lstrip(
string_example.encode( string_example.maketrans(
string_example.endswith( string_example.partition(
string_example.expandtabs( string_example.replace(
string_example.find( string_example.rfind(
string_example.format( string_example.rindex(
string_example.format_map( string_example.rjust(
string_example.index( string_example.rpartition(
string_example.isalnum( string_example.rsplit(
string_example.isalpha( string_example.rstrip(
string_example.isdecimal( string_example.split(
string_example.isdigit( string_example.splitlines(
string_example.isidentifier( string_example.startswith(
string_example.islower( string_example.strip(
string_example.isnumeric( string_example.swapcase(
string_example.isprintable( string_example.title(
string_example.isspace( string_example.translate(
string_example.istitle( string_example.upper(
string_example.isupper( string_example.zfill(
One of the methods in the list is the split()
method.
>>> string_example.split()
['This', 'is', 'a', 'great', 'thing']
>>> for word in string_example.split():
... print(word)
...
This
is
a
great
thing
split_lines()
method looks for a '\n'
, and it
divides the string based on it. If you have a string which contains
newlines, and you would want to divide it into a number of strings with
each line as a new element, the method you can use is
split_lines()
.
>>> string_example = "This\nis\n\ngreat\nthing"
>>> print(string_example)
This
is
great
thing
>>> string_example = "This\nis\na\ngreat\nthing"
>>> print(string_example)
This
is
a
great
thing
>>> string_example.splitlines()
['This', 'is', 'a', 'great', 'thing']
>>>
The last thing which we look at, is concatenation operator.
>>> 1 + 2
3
>>> "1" + "2"
'12'
>>> "1" + 1
TypeError: must be str, not int
>>> "ABC" + "DEF"
'ABCDEF'
In Python, you cannot do +
operator between two different
types. +
with two strings is concatenation.
+
with two numbers is addition.
One other interesting operator on strings is multiplication. If you do a
'1' * 20
, What do you think will be the output?
>>> 1 * 20
20
>>> '1' * 20
'11111111111111111111'
>>> 'A' * 10
'AAAAAAAAAA'
If you multiply a string with number
, the string value is
concatenated number
times.
The last thing which we look at in this step, is comparing strings.
Let’s say we have a string with a value str = 'test'
, and you
have another string to with a value str1 = 'test1'
.
We want to check whether both these strings are the same.
>>> str = "test"
>>> str2 = "test1"
>>> str == str2
False
>>> str2 = "test"
>>> str == str2
True
You can compare strings using the ==
operator.
In this step, we explored a few exercises on strings, covering areas such as:
+
*
==
operator to compare strings
Welcome to the section on Loops. In this section, we will look at a
variety of loops that are available in Python. We will look mainly at the
for
loop, and the
while
loop.
Let’s start with revising the basics of the for loop, we have learned in the previous steps.
We saw that a for
loop helps us to loop around the same set
of code statements, many times over.
Let’s look at a few simple examples, once again.
The syntax of a for
loop is very simple.
For example, this code snippet will tell you all about it:
for i in range(1, 11): print(i)
.
What does this do? Very simple, it prints from 1
to
10
.
In the call to the range()
function, the second parameter is
exclusive. We are actually looping from 1
to 10
,
and this piece of code, print(i)
, is being executed for
different values of i
.
>>> for i in range(1,11):
... print(i)
...
1
2
3
4
5
6
7
8
9
10
for
loop can also be used to loop round the characters in a
string.
>>> for ch in "Hello World":
... print(ch)
...
H
e
l
l
o
W
o
r
l
d
for
loop can be used to loop around all the words in a given
sentence.
>>> for word in "Hello World".split():
... print(word)
...
Hello
World
for
loop can be used to loop around a specific list of
values.
>>> for item in (3, 6, 9):
... print(item)
...
3
6
9
In this step, we started with discussing and revising basic concepts about
the for
loop
Welcome back to this step, where we would do a lot of exercises with the
for
loop.
The first exercise is to find out if a number is prime. We want to
write a method, is_prime()
, which accepts an integer
value as parameter, and returns whether it’s a prime.
(Hint: A prime number is something which is only
divisible by 1
and itself).
5
is only divisible by 1
and
5
. It is not divisible by any other number. Same is the
case with 7
and 11
.
6
is divisible by 1
,
2
, 3
and 6
. So it’s not a
prime number.
The second exercise is to write a method to calculate the sum up to a
given integer, starting from 1
. Hint: If
I would want to find that the sum up to 6
. what’s needed
is 1 + 2 + 3 + 4 + 5 + 6
.
The third exercise is to find that the sum of divisors of a given
integer. Hint: Let’s say we want to find out the sum
of the divisors of 15
. The divisors of
15
are 1
, 3
, 5
and
15
. So I would want to calculate
1 + 3 + 5 + 15
, and return that value.
Fourth exercise is to print a numbered triangle, when given a specific integer.
Hint: Given an input 5
, we would want to print the number
triangle of these kind:
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5.
These are the exercises for the for
loop. We also test our
skills, with creating method and executing them, in our IDE.
Let’s start with creating the is_prime()
method, in a file
named for_exercises
.
We would want to accept an int
parameter, and find out if it
is prime, or not.
We need to check whether it’s divisible by any other number, other than
1
and itself. If we are passed in a value of 5
,
you want to see if it’s divisible by any of 2
,
3
or 4
.
def is_prime(number):
We can use a for
loop. We can structure it like this:
for divisor in range(1, number): ...
. We would not want to
divide it with 1
, but start with 2
instead, and
go up to number-1
, which is 4
.
for divisor in range(2,number):
How can we check if the number
is divisible by
divisor
?
By using the %
operator. If number
is divisible
by divisor
we return False
.
for divisor in range(2,number):
if number % divisor == 0:
return False
What happens if the code comes up to the end? It would mean we tried with
2
, 3
and 4
, but
number
was not divisible by all of them. In that case,
number
would be prime, and we can safely return
True
.
for divisor in range(2,number):
if number % divisor == 0:
return False
return True
For 1
, the rules are a little different, as it is neither a
prime or composite. We will add an if
condition to check if
the number is 1
. if(number < 2):
This if
condition is called a guard check or a boundary
check, to make sure that you are processing only the right input. If
number
has a value less than 2
, do nothing. OK,
it’s not a prime.
Here is the entire code at one place, for your reference:
def is_prime(number):
if(number < 2):
return False
for divisor in range(2,number):
if number % divisor == 0:
return False
return True
print(is_prime(5));
In the previous step, we looked at solving the
is_prime()
exercise. In this step, let’s look at an
implementation of sum_up_to_n()
. Here is the entire code for
this exercise:
def sum_upto_n(number):
sum = 0
for i in range(1, number+1):
sum = sum + i
return sum
print(sum_upto_n(6))
print(sum_upto_n(10))
In this step, we:
1
, up to the input integer n
.
Let’s focus on the third exercise, sum_of_divisors
.
One of the clues we can give you, is that
sum_of_divisors()
is very similar to is_prime()
.
You want to find out if a number is dividing 15
, and if it’s
dividing 15
, with the remainder of 0
, then you
need to add that up.
def calculate_sum_of_divisors(number):
sum = 0
if(number < 2):
return sum
for divisor in range(1,number+1):
if number % divisor == 0:
sum = sum + divisor
return sum
print(calculate_sum_of_divisors(6))
print(calculate_sum_of_divisors(15))
In this step, Let’s look at the last exercise -
print_a_number_triangle
.
For example, if we call such a function with input 5
, the
output needs to be:
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
Let start with a simple thing. Let’s try and print
1 2 3 4 5
first, and then we would look at how to print the
rest of the output. Lets proceed with defining this method.
We can say def print_a_number_triangle(number): ...
that
takes a number as an input. You want to print a sequence of integers
starting from 1
, up to that specific number
. How
can you do that? Let’s try this:
for i in range(1,number+1): print(i)
What would happen? Let’s
call print_a_number_triangle(5)
now. It prints:
1
2
3
4
5
on individual lines.
To print this sequence on a single line, let’s delimit them with
<SPACE>
instead. Call print()
like this
instead: for i in range(1,number+1): print(i, end=" ")
.
Let’s see what would happen now. 1 2 3 4 5
To solve our exercise, we want to repeat this again and again.
Yes, we need another for loop around it!
for j in range(1, number+1):
for i in range(1, number + 1):
print(i, end=" ")
Make sure that you have the indentation right. This is called
loop within a loop
.
The output of above program is
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
Let’s add print("\n")
, so we have a new line at the end of
each outer loop iteration.
for j in range(1, number+1):
for i in range(1, number + 1):
print(i, end=" ")
print("\n")
Output
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
We are printing a square, not a triangle.
What we want to do is to print up to 1
in first line, upto
2
in second line and so on.
How can we do that? Think about it.
When you are inside this loop, you can see the variable j
.
Instead of number+1
, let’s say j + 1
.
When j
has a value of 1
, for
will
print from 1
to 1
. When j
has a
value of 2
, print from 1
to 2
,
literally printing 1 2
. When j has a value of 3
,
I’ll print from 1
to 3
. Let’s try this and see
what would happen.
for j in range(1, number+1):
for i in range(1, j + 1):
print(i, end=" ")
print("\n")
You can see that our number triangle is ready!
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
Here is the entire code for you:
def print_a_number_triangle(number):
for j in range(1, number + 1):
for i in range(1, j + 1):
print(i, end=' ')
print()
print_a_number_triangle(6)
An important point to note is, a couple of these things can be done in a much simpler way. We will look at these options when we talk about functional programming.
In this step, we:
Let’s look at one of the other loops which is present in Python, called
the while
loop.
In the for
loop, we can specify the range of our iteration,
by using the range()
function.
In a while
loop, we specify a logical condition. While the
condition is true, loop continues running.
Do you remember one place where we use the condition until now? It was in
an if
statement.
Let’s see how to use a simple while
loop.
>>> i = 5
>>> if i == 5:
... print("i is 5")
...
i is 5
Let’s say i
has a value of 0
, and we then do:
while i < 5: print(i)
.
>>> i = 0
>>> while i < 5:
... print(i)
...
0
0
0
0
0
0
0
0
^CTraceback (most recent call last):
File "<stdin>", line 2, in <module>
KeyboardInterrupt
>>>
KeyboardInterrupt
If we leave it to run, you’d see that it continuously prints
0
again, and again. Let’s do a <CTRL-C>
or
<COMMAND-C>
to interrupt this.
What is happening here?
Initially i
is 0
, and the condition
i < 5
is True
, and print(i)
is
executed. Next iteration, it checks the condition, it is
True
, and 0
is printed. This continues to
happen. What’s happening is an infinite loop.
One of the important things to make sure in a while
loop, is
to increment the value of i
. We need to say something like
i = i + 1
.
>>> while i < 5:
... print(i)
... i = i + 1
...
0
1
2
3
4
So how does it work? *i
initially had a value of
0
. First the condition is checked. It’s True
, so
0
is printed and then the value of i
is
incremented to 1
.
i
is still less than 5
, so the loop continues
to execute, and this happens until 4
is printed.
i
again gets incremented to 4 + 1
, or
5
.
i < 5
. This is now
False
. Control goes out of the while
loop, and
terminates it.
When executing a while
, control flow is just based on a
condition. As long as the condition is True
, we keep
executing the code. An important thing to remember, is to make sure the
control variable is updated.
>>> for i in range(0,5): print(i)
...
0
1
2
3
4
A for
loop is much simpler to code than a while
.
With while
, we have to write an expression statement, to
increment the value.
The question you might have is - What are the situations when you should use a while?
We will look at that very soon.
In this video, we:
while
loop in Python
while
, and a
for
loop
In the previous step, we were introduced to while
loop. In
this step, let’s look at a couple of exercises using the
while
loop.
print_squares_upto_limit(30)
: We need to print all the
squares of numbers, up to a limit of 30
. The output needs
to be 1 4 9 16 25
.
print_cubes_upto_limit(30)
: We need to print all the
cubes of numbers, up to a limit of 30
.The output needs to
be 1 8 27.
Here is the entire code for your reference:
def print_squares_upto_limit(limit):
i = 1
while i * i < limit:
print(i*i, end = " ")
i = i + 1
Now the next exercise, was to print cubes up to a limit.
The expression in the while
condition should now be
i*i*i < 30
.
def print_cubes_upto_limit(limit):
i = 1
while i * i * i < limit:
print(i*i*i, end = " ")
i = i + 1
print_cubes_upto_limit(80)
Could we have implemented above two examples with for
loop?
It would’ve been a little more difficult.
Typically, we use a for
loop when we know how many times the
loop will be executed is clear at the start.
If we do not know, how many times a loop will run, while
is a
better option.
Earlier we used if
statement to implement a solution for
this:
We would want to enhance it to execute in a loop multiple times, until the user chooses to exit. We will add an option 5 - Exit.
Here’s the earlier code we wrote with if:
number1 = int(input("Enter Number1: "))
number2 = int(input("Enter Number2: "))
print("\n\n1 - Add")
print("2 - Subtract")
print("3 - Divide")
print("4 - Multiply")
choice = int(input("Choose Operation: "))
# print(number1 + number2)
# print(choice)
if choice==1:
result = number1 + number2
elif choice==2:
result = number1 - number2
elif choice==3:
result = number1 / number2
elif choice==4:
result = number1 * number2
else:
result = "Invalid Choice"
print(result)
Let’s use while
around the if
. We cannot predict
what choice the user enters. We’ll say:
while choice != 5:...
and at the end, we would want to ask
the user for a choice. If your choice is anything other than
5
, we will give the output, and print the whole menu again.
number1 = int(input("Enter Number1: "))
number2 = int(input("Enter Number2: "))
print("\n\n1 - Add")
print("2 - Subtract")
print("3 - Divide")
print("4 - Multiply")
print("5 - Exit")
choice = int(input("Choose Operation: "))
while(choice != 5):
# print(number1 + number2)
# print(choice)
if choice==1:
result = number1 + number2
elif choice==2:
result = number1 - number2
elif choice==3:
result = number1 / number2
elif choice==4:
result = number1 * number2
else:
result = "Invalid Choice"
print(result)
choice = int(input("Choose Operation: "))
print("Thank You")
These are the kind of situations where while
loop is good. W
In this step, we:
while
loop was ideal to solve this exercise
We would be discussing about a wide variety of puzzles related to
for
loop, and while loop
.
What would be the output of this script?
for i in range(1,11,2):
print(i, end=' ')
Output : 1 3 5 7 9
`range()
function accept astep
parameter. It
starts with1
, and at each step it increments by
2
.11
is exclusive. So`11``` is not printed.
Let’s look at the next one.
for i in range(11,0,-1):
print(i, end=' ')
Output : 1 10 9 8 7 6 5 4 3 2 1
On to the next puzzle.
i = 5
while i*i < 10:
print(i)
print("done")
Output : done
On to the next puzzle.
i = 2
while i * i < 10:
print(i, end=" ")
print('done')
Output :<PHONE_NUMBER>22....
It’s printing 2
continuously. We forgot the increment. Let’s
try and fix this.
i = 2
while i*i < 10:
print(i, end=' ')
i = i + 1
print("done")
Output : 2 3 'done'
.
Now, let’s look at a new puzzle, with a new key word, break.
for i in range(1,11):
if i==5:
break
print(i, end=' ')
print("done")
Output : 1 2 3 4 'done'
break
breaks out of the loop when executed. With
5
, the condition matches if i==5
.
break
is executed, and we get outside the
for
loop.
Let’s look at the next puzzle:
for i in range(1,11):
if i%2:
break
print(i , end=' ')
print("done")
Output : 'done'
You don’t really see anything being printed from the for loop.
When you evaluate 1 % 2
, it returns 1
as the
remainder. Any non-zero number is considered to be True
. We
break out of the loop.
Next Puzzle:
for i in range(2,11):
if i%2:
break
print(i , end=' ')
print("done")
It prints 2
, because 2 % 2
is 0
,
and 0
is False
, so the print is executed as the
break
is not called. However when it becomes 3
,
3 % 2
is 1, which is considered to be True
. The
break
is executed, and we exit out of the loop.
Let’s look at a new keyword, called continue. Look at the following code:
for i in range(1,11):
if i%2==0:
continue
print(i , end=' ')
print("done")
It’s printing 1 3 5 7 9 'done'
What do you think will be the output?
continue
skips the code inside the for
loop for
the current iteration, and then resumes the next one.
Lines after continue
in loop are skipped and you’d start the
next iteration of the loop.
What would happen, if we remove the i%2 == 0
, and only have
condition as i%2
?
for i in range(1,11):
if i%2:
continue
print(i , end=' ')
print("done")
It prints even numbers only. For odd numbers, i%2
is
True
. They are skipped.
The last puzzle we’ll be looking at is exactly a modification of the
earlier one. Instead of. i%2
, we give the condition as
i % 2 != 0
.
for i in range(1,11):
if i%2!=0:
continue
print(i , end=' ')
print("done")
What do you think will be the output? Yes you’re right. No change in
output, because if i % 2 !=0
is the same as a
if i % 2
. That’s because if i % 2
is
0
, then i % 2
will be False
. If
i % 2
is 0
, then i % 2 != 0
also
will be False
.
There are a number of predefined modules in Python, providing a wide variety of features for use. In this step, let’s learn how to import a module, and how to use methods from specific modules.
Let’s talk about the math
module
to start off
with. Import it by saying import math
. If you do
math.
and press <TAB>
, it shows all the
functions that are defined in the math
module
.
>>> import math
>>> math.
math.acos( math.erf( math.inf math.pi
math.acosh( math.erfc( math.isclose( math.pow(
math.asin( math.exp( math.isfinite( math.radians(
math.asinh( math.expm1( math.isinf( math.sin(
math.atan( math.fabs( math.isnan( math.sinh(
math.atan2( math.factorial( math.ldexp( math.sqrt(
math.atanh( math.floor( math.lgamma( math.tan(
math.ceil( math.fmod( math.log( math.tanh(
math.copysign( math.frexp( math.log10( math.tau
math.cos( math.fsum( math.log1p( math.trunc(
math.cosh( math.gamma( math.log2(
math.degrees( math.gcd( math.modf(
math.e math.hypot( math.nan
math.floor(4.5)
gives you back 4
.
If you want to find out a little bit more about the
math.floor()
, say help(math.floor)
.
This command would show you the documentation for this
floor()
function. It says “floor(x) returns the floor of x as
an integer. This is the largest integer, which is less than or equal to
x”.
>>> math.floor(4.5)
4
>>> help(math.floor)
>>> help(math)
You can import all the members from the entire module by using
from math import *
.
What would happen now, is all the functions which are present in
math
, are visible in your namespace.
You can directly call floor(5)
, without the need for
math.floor()
.
You can do gcd(2, 4)
, where gcd()
finds the
greatest common divisor of these two numbers.
For help, You can use help(gcd)
, and it responds with the
documentation.
An important things to remember, is when you do
import * from math
, you are importing everything from there.
This means if you have any local variable with the names as these
functions, they might get shadowed.
>>> from math import *
>>> floor(5)
5
>>> gcd(34,56)
2
Typically it’s not really considered to be a good practice to import everything into the namespace.
If there are specific things that you would want to import and use them
directly, Then you can import just that, by doing this:
from math import floor
. Also
from math import gcd
.
>>> from math import gcd
>>> gcd(56,68)
4