User-Defined Functions in Python

User-Defined Functions

A function is a set of statements that perform a specific task, a common structuring element that allows you to use a piece of code repeatedly in different parts of a program. The use of functions improve a program’s clarity and comprehensibility and makes programming more efficient by reducing code duplication and breaking down complex tasks into more manageable pieces. Functions are also known as routines, subroutines, methods, procedures, or subprograms.

They can be passed as arguments, assigned to variables, or stored in collections.

A user-defined Python function is created or defined by the def statement and follows the syntax:

def function_name(parameter list):

function body/statements

The indented statements make up the body of the function and are executed when the function is called. Once the function is called, parameters inside round brackets become arguments.

Function bodies can have more than one return statement which may be placed anywhere within the function block. Return statements end the function call and return the value of the expression after the return keyword. A return statement with no expression returns the special value ‘None’. In the absence of a return statement within the function body, the end of the function is indicated by the return of the value ‘None’.

The docstring is an optional statement after the function title which explains what the function does. While it is not mandatory, documenting your code with a docstring is a good programming practice.

Here is a simple function that prints I love Pizza!

def love_pizza():

print “I love Pizza!”

Here is a function with a parameter and return keyword:

def absolute_value(number):

if number >= 0:

return number

else:

return -number

print(absolute_value(3))

print(absolute_value(-5))

In the above example, number is the parameter of the function absolute_value. It acts as a variable name and holds the value of a passed in argument.

Here is the output when the above code is run:

3

5

Following is a function with an if-then-else statement.

def shutdown(yn):

if yn.lower() == “y”:

return(“Closing files and shutting down”)

elif yn.lower() == (“n”):

return(“Shutdown cancelled”)

else:

return(“Please check your response.”)

print(shutdown(“y”))

print(shutdown(“n”))

print(shutdown(“x”))

Python Shell will display:

Closing files and shutting down

Shutdown cancelled

Please check your response.

Function can take more than one parameter and use them for computations:

def calculator(x, y):

return x * y + 2

print(calculator(2,6))

print(calculator(3,7))

Run the code and you’ll get the output:

14

23

Functions can call other functions

Functions can perform different types of actions such as do simple calculations and print text. They can also call another function.

For example:

def members_total(n):

return n * 3

def org_total(m):

return members_total(m) + 5

To see what you code does, enter the following print commands:

print(org_total(2))

print(org_total(5))

print(org_total(10))

You’ll get these results:

11

20

35