Python Functions

Difficulty Level : Easy
Last Updated : 23 Aug, 2021

Python Functions is a block of related statements designed to perform a computational, logical, or evaluative task. The idea is to put some commonly or repeatedly done tasks together and make a function so that instead of writing the same code again and again for different inputs, we can do the function calls to reuse code contained in it over and over again.

Functions can be both built-in or user-defined. It helps the program to be concise, non-repetitive, and organized.

Syntax:

def function_name(parameters):
    """docstring"""
    statement(s)
    return expression

Creating a Function

We can create a Python function using the def keyword.

Example: Python Creating Function

Python3

# A simple Python function
 
def fun():
  print("Welcome to GFG")

Calling a Function

After creating a function we can call it by using the name of the function followed by parenthesis containing parameters of that particular function.

Example: Python Calling Function

Python3

# A simple Python function
 
def fun():
  print("Welcome to GFG")
         
# Driver code to call a function
fun()

Output

Welcome to GFG

Arguments of a Function

Arguments are the values passed inside the parenthesis of the function. A function can have any number of arguments separated by a comma.

Example: Python Function with arguments

In this example, we will create a simple function to check whether the number passed as an argument to the function is even or odd.

Python3

# A simple Python function to check
# whether x is even or odd
 
 
def evenOdd(x):
    if (x % 2 == 0):
        print("even")
    else:
        print("odd")
 
 
# Driver code to call the function
evenOdd(2)
evenOdd(3)

Output

even
odd

Types of Arguments

Python supports various types of arguments that can be passed at the time of the function call. Let’s discuss each type in detail.

Default arguments

A default argument is a parameter that assumes a default value if a value is not provided in the function call for that argument. The following example illustrates Default arguments.

Python3

# Python program to demonstrate
# default arguments
 
 
def myFun(x, y=50):
    print("x: ", x)
    print("y: ", y)
 
 
# Driver code (We call myFun() with only
# argument)
myFun(10)

Output

('x: ', 10)
('y: ', 50)

Like C++ default arguments, any number of arguments in a function can have a default value. But once we have a default argument, all the arguments to its right must also have default values.

Keyword arguments

The idea is to allow the caller to specify the argument name with values so that caller does not need to remember the order of parameters.

Python3

# Python program to demonstrate Keyword Arguments
def student(firstname, lastname):
    print(firstname, lastname)
 
 
# Keyword arguments
student(firstname='Geeks', lastname='Practice')
student(lastname='Practice', firstname='Geeks')

Output

('Geeks', 'Practice')
('Geeks', 'Practice')

Variable-length arguments

In Python, we can pass a variable number of arguments to a function using special symbols. There are two special symbols:

*args (Non-Keyword Arguments)
**kwargs (Keyword Arguments)

Example 1: Variable length non-keywords argument

Python

# Python program to illustrate
# *args for variable number of arguments
 
 
def myFun(*argv):
    for arg in argv:
        print(arg)
 
 
myFun('Hello', 'Welcome', 'to', 'GeeksforGeeks')

Output

Hello
Welcome
to
GeeksforGeeks

Example 2: Variable length keyword arguments

Python3

# Python program to illustrate
# *kargs for variable number of keyword arguments
 
 
def myFun(**kwargs):
    for key, value in kwargs.items():
        print("%s == %s" % (key, value))
 
 
# Driver code
myFun(first='Geeks', mid='for', last='Geeks')

Output

first == Geeks
mid == for
last == Geeks

Docstring

The first string after the function is called the Document string or Docstring in short. This is used to describe the functionality of the function. The use of docstring in functions is optional but it is considered a good practice.

The below syntax can be used to print out the docstring of a function:

Syntax: print(function_name.__doc__)

Example: Adding Docstring to the function

Python3

# A simple Python function to check
# whether x is even or odd
 
 
def evenOdd(x):
    """Function to check if the number is even or odd"""
     
    if (x % 2 == 0):
        print("even")
    else:
        print("odd")
 
 
# Driver code to call the function
print(evenOdd.__doc__)

Output

Function to check if the number is even or odd

The return statement

The function return statement is used to exit from a function and go back to the function caller and return the specified value or data item to the caller.

Syntax: return [expression_list]

The return statement can consist of a variable, an expression, or a constant which is returned to the end of the function execution. If none of the above is present with the return statement a None object is returned.

Example: Python Function Return Statement

Python3

def square_value(num):
    """This function returns the square
    value of the entered number"""
    return num**2
 
 
print(square_value(2))
print(square_value(-4))

Output:

4
16

Is Python Function Pass by Reference or pass by value?

One important thing to note is, in Python every variable name is a reference. When we pass a variable to a function, a new reference to the object is created. Parameter passing in Python is the same as reference passing in Java.

Example:

Python3

# Here x is a new reference to same list lst
def myFun(x):
    x[0] = 20
 
 
# Driver Code (Note that lst is modified
# after function call.
lst = [10, 11, 12, 13, 14, 15]
myFun(lst)
print(lst)

Output

[20, 11, 12, 13, 14, 15]

When we pass a reference and change the received reference to something else, the connection between the passed and received parameter is broken. For example, consider the below program.

Python3

def myFun(x):
 
    # After below line link of x with previous
    # object gets broken. A new object is assigned
    # to x.
    x = [20, 30, 40]
 
 
# Driver Code (Note that lst is not modified
# after function call.
lst = [10, 11, 12, 13, 14, 15]
myFun(lst)
print(lst)

Output

[10, 11, 12, 13, 14, 15]

Another example to demonstrate that the reference link is broken if we assign a new value (inside the function).

Python3

def myFun(x):
 
    # After below line link of x with previous
    # object gets broken. A new object is assigned
    # to x.
    x = 20
 
 
# Driver Code (Note that lst is not modified
# after function call.
x = 10
myFun(x)
print(x)

Output

Exercise: Try to guess the output of the following code.

Python3

def swap(x, y):
    temp = x
    x = y
    y = temp
 
 
# Driver code
x = 2
y = 3
swap(x, y)
print(x)
print(y)

Output

2
3

Anonymous functions:

In Python, an anonymous function means that a function is without a name. As we already know the def keyword is used to define the normal functions and the lambda keyword is used to create anonymous functions. Please see this for details.

Python3

# Python code to illustrate the cube of a number
# using lambda function
 
 
def cube(x): return x*x*x
 
cube_v2 = lambda x : x*x*x
 
print(cube(7))
print(cube_v2(7))

Output

Python Function within Functions

A function that is defined inside another function is known as the inner function or nested function. Nested functions are able to access variables of the enclosing scope. Inner functions are used so that they can be protected from everything happening outside the function.

Python3

# Python program to
# demonstrate accessing of
# variables of nested functions
 
def f1():
    s = 'I love GeeksforGeeks'
     
    def f2():
        print(s)
         
    f2()
 
# Driver's code
f1()

Output

I love GeeksforGeeks

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