Python Functions

Created: February-13, 2018

In this section, you will learn

What is a function
How to create a function
Types of functions

What is Python Function

A function is a small block of a program which contains a number of statements to perform a specific task. When you have a program of thousands of lines performing different tasks then you should divide the program into small modules (blocks) which increases the readability and lowers the complexity.

How to Define a Python Function

The following is the syntax to define a function:

def functionName(arguments):
    """This is the definition of this function"""
    statements
    return returnParam

The keyword def is used to define a function.
functionName is the name of the function.
arguments are optional. Arguments provide values to function to perform operations on.
Colon (:) ends the function header.
"""This is the definition of this function""" is a docstring and is optional which describes what the function does.
statements refers to the body of the function.
The statement return optionally returns the result to the caller.

Python Function Example:

def language(p):
    """Function to print a message"""
    print("Programming language:", p)

Here a function language is defined which has one argument p passing from the caller. Inside the function, there is a docstring and a print statement to print the message containing the argument p.

Call a Function in Python:

A function can be called from anywhere in the program. A function can be called by its name with the required parameters.

language('Python')

Programming language: Python

The `return` Statement:

The return statement transfers the control back to the codes where the function is called. It indicates the ending of the function definition.

The syntax of return is as follows:

return [values_to_be_returned]

If there is no return statement in a function, a None object will be returned.

Example of Using `return` Statement:

def square(n):
    return n*n
print("Square of 4=", square(4))

Square of 4=16

In this code, the function is called in a print function and 4 is passed as an argument. Inside the function, you have an expression n*n which is evaluated and the result is returned to where the function is called (print statement).

Scope and Lifetime of Python Variables:

The scope of a variable is where a variable can be accessed. When a variable is declared inside a function, it is not accessible to the outside of that function. This type of variables is called local variable and is accessed only to the function where it is declared.

The lifetime of a variable is the time during which a variable exists in memory. When a variable is declared inside a function, the memory of it will be released when the control jumps out of the function.

See the example below:

def fun():
    a = 12
    print("Value of a inside function:", a)
a = 24
fun()
print("Value of a outside function:", a)

Value of a inside function: 12
Value of a outside function: 24

In this code, variable a inside the function and variable a outside the function are different variables.

If you try to access variables declared inside functions from outside, you will come across an error - NameError name 'x' is not defined. But the variables declared outside of functions have a global scope and can be accessed from inside.

Types of Functions:

Functions in Python can be categorized into two types:

Built-in functions: have a predefined meaning and perform specific tasks.
User-defined functions: are defined by the user containing any number of statements to perform user-defined tasks.