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Functions in Python


A function is a named sequence of statement(s) that performs a computation. It contains line of code(s) that are executed sequentially from top to bottom by Python interpreter.
Instead of writing a large program we can write small functions as a specific part of program to accomplish the task.
They are the most important building blocks for any software in Python.
Functions can be categorized as belonging to
i. Modules
ii. Built in
iii. User Defined

User defined Function

It is also possible for programmer to write their own function(s). These functions can then be combined to form a module which can then be used in other programs by importing them.

• To define a function keyword def is used.
• After the keyword comes an identifier i.e. name of the function, followed by parenthesized list of parameters and the colon which ends up the line.
• Next follows the block of statement(s) that are the part of function. Statements in a block are written with indentation.

Advantages of Functions:

• Creating a new function gives you an opportunity to name a group of statements, which makes your program easier to read, understand and debug.
• Functions can make a program smaller by eliminating repetitive code. Later, if you make a change, you only have to make it in one place.
• Dividing a long program into functions allows you to debug the parts one at a time and then assemble them into a working whole.
• Well-designed functions are often useful for many programs. Once you write and debug one, you can reuse it.

Define a Function

the Syntax of function is:


# indented (4 space) statement


Note: The first line of function definition, i.e., Line No. 1 is called header and the rest, i.e. Line No. 2 in our example, is known as body.
Consisting of sequence of indented (4 space) Python statement(s), to perform a task.

Let‟s write a function to greet the world:
def sayHello (): # Line No. 1
    print( “Hello World!”) # Line No.2

Types of Functions


There are various types of functions variations:

1. User-defined Functions without argument and without return statement
2. User-defined Functions with argument and without return statement
3. User-defined Functions with argument and with return statement
4. User-defined Functions with argument and with multiple return values

User-defined Functions without argument  and without return statement


def evenodd():

     a=int(input(“enter a number:”)
              if a%2==0:

                    print( a,”is even“)


                    print( a,”is odd“)



Enter a number:45
45 is odd

User-defined Functions with argument and without return statement


def evenodd(a):
      if a%2==0:

            print( a,”is even“)


           print( a,”is odd“)



>>> evenodd(45)
45 is odd
>>> evenodd(66)
66 is even

User-defined Functions with argument and with return statement


def evenodd(a):
    if a%2==0:
           return ” even”
          return ” odd”

>>> x=evenodd(34)
>>> print( x )

User-defined Functions with argument and with multiple return statement


def calc (a, b) :


   mul=a *b


    return add, sub, mul, div


x = int (input ( “Enter a number1”)) 

y = int (input ( “Enter a number2”)) 


print( “sum is “,a)

print( “difference is”,s)

print( “multiplication”,m)

print( “quotient is”, d)

Parameters and Arguments in Functions

• When we write header of any function then the one or more values given to its parenthesis ( ) are known as parameter.
• These are the values which are used by the function for any specific task. While argument is the value passed at the time of calling a function. 
• Formal parameters are said to be parameters and actual parameters are said to be arguments.



1. Positional Arguments
2. Default Arguments
3. Keyword Arguments
4. Variable length Arguments


1. Positional Arguments

The arguments are passed in correct positional order to function according to the variables in function header.

def subtract(a,b):


>>> subtract (200, 100)
>>> subtract (100, 200)


2. Default Arguments

This is point to remember This is point to that the default should be given aft default argument.

A default argument is a function parameter that has a default value provided to it. If the user does not supply a value for this parameter, the default value will be used. If the user does supply a
value for the default parameter, the user-supplied value is used. 
def greet (message, times=1):
      print message * times
>>> greet (‘Welcome’) # function call with one argument value
>>> greet (‘Hello’, 2) # function call with both the argument values.




3. Keyword Argument

• This is explicit type of assigning the values to function variable, where we use name of the variable and its value.
• If we change the order of variables still it makes no difference in result.

>>> def wish (name,msg) :
              print (“Hello! ” , name, “! ” ,msg, “! “)

wish (name=”Ram” , msg=”Good Evening”)
Hello! Ram ! Good Evening !

wish (msg=”Good Evening” , name=” Ram”)
Hello! Ram ! Good Evening !



4. Variable Length Arguments


• Here any number of Arguments can be passed to function.
• Special symbol * asterik is used to declared it as variable length argument.

def sum(*n):


      for i in n:

      print (“The sum = ” , s)


>>>sum ( )
sum =0


sum = 10

>>>sum (10, 20)
sum = 30


>>>sum (10, 20, 30)
sum =60

Scope of a variables

Variables can only reach the area in which they are defined, which is called scope.
Think of it as the area of code where variables can be used.
There are two types of variable scopes :
● Global Scope
● Local Scope

Python supports global variables (usable in the entire program) and local variables. By default, all variables declared in a function are local variables.


Global Scope

With global scope, variable can be used anywhere in the program

def test ( ):
        print(“inside test x is “, x)

print(“value of x is “, x)


inside test x is 50
value of x is 50


Local Scope

With local scope, variable can be used only within the function / block that it is created
def test ( ):
      y = 20 
      print(‘value of x is ’, X, ‘ y is ’, y)

test( ) 

print(‘value of x is ’, X, ‘ y is ‘, y)


On executing the test( ) we will get
Value of x is 50 y is 20

The next print statement will produce an error, because the
variable y is not accessible outside the def()



To access global variable inside the function prefix keyword global with the variable
def test ( ):
    global x =5
     y =2
print(‘value of x & y inside the function are ‘ , x , y)

print(‘value of x outside function is  ‘, x)


This code will produce following output:
Value of x & y inside the function are 5 2
Value of x outside the function is 5

Unsolved questions for Assignment:


1. Python Program to Calculate the Average of Numbers in a Given List
2. Python Program to Exchange the Values of Two Numbers Without Using a Temporary Variable
3. Python Program to Read a Number n and Compute n+nn+nnn
4. Python Program to Reverse a Given Number
5. Python Program to Check Whether a Number is Positive or Negative
6. Python Program to Take in the Marks of 5 Subjects and Display the Grade
7. Python Program to Print all Numbers in a Range Divisible by a Given Number
8. Python Program to Read Two Numbers and Print Their Quotient and Remainder
9. Python Program to Accept Three Digits and Print all Possible Combinations from the Digits
10. Python Program to Print Odd Numbers Within a Given Range
11. Python Program to Find the Sum of Digits in a Number
12. Python Program to Find the Smallest Divisor of an Integer
13. Python Program to Count the Number of Digits in a Number
14. Python Program to Check if a Number is a Palindrome
15. Python Program to Print all Integers that Aren’t Divisible by Either 2 or 3 and Lie between 1 and 50.
16. Python Program to Read a Number n And Print the Series “1+2+…..+n= “
17. Python Program to Read a Number n and Print the Natural Numbers Summation Pattern
18. Python Program to Print an Identity Matrix
19. Python Program to Print an Inverted Star Pattern
20. Python Program to Read Print Prime Numbers in a Range using Sieve of Eratosthenes

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