Functions in Python

Updated on May 20, 2019
Sam Shepards profile image

I'm a software developer with a great interest in data analysis and statistics.

Functions are blocks of code that are encapsulated under a function name so that they can be executed by calling the function name. Functions in Python can be of two types – built-in and user-defined functions.

Built-in functions are those which come along with python installation as libraries. Print(), input(), list() and tuple() are some of built-in functions that we saw so far. User-defined functions are those which we create in our programs so that a particular block of code can be reused.

Syntax of a function looks like:

def function_name( input_params ):
   "function_docstring" # documentation string
   # body of function goes here
   return [return_value] # returns an optional value

The following are the important parts of a function:

  • The keyword def denotes the starting point of a function block.
  • The def keyword is followed by function name and parentheses and a colon. The colon and indentation of the code that follows denote the start and end of a function body
  • If there are input parameters that need to be passed so that the function performs some operations, they are put inside the parentheses.
  • The function_docstring represents documentation for the given function. It is not mandatory to have documentation string for every function.
  • After optional docstring, the body of the function is added. As mentioned earlier, the body needs to have proper indentation to be regarded as part of the function.
  • The function body can end with a return statement that can return a value if required. It is not mandatory to always return something from a function.

Once a function is created, it can be called using function_name(params_optional) inside another function or from the command line of python interpreter.

Defining a Function

Consider the following example:

def printMe( x ):
    print("My value is : ", x)
def sayHello( name ):
   "function which says hello" 
   print( "Hello, ", name, "!" )
def getMySquare( x ):
    return ( x*x )
print( "Square of 12 = ",getMySquare( 12 ) )

The function printMe demonstrates how to have a simple function with only one line and no return value. As soon as the indentation changes, python interpreter understands that the function has ended.

The function sayHello has a return statement at the end. This is similar to the break statement we saw in control flow. It just returns from the function. GetMySquare is a function where a proper value is returned. Note that function docstring is ignored by python interpreter and it doesn’t alter the output.

The output for the above program will be:

My value is : 12
Hello, Sam !
Square of 12 = 144

Specifying Default Parameters

It is possible to specify default input parameters for a function. This can be handy if the caller prefers to have fewer parameters passed as arguments.

def printinfo( name, country = "India" ):
   "This prints name and country information"
   print ( "Name: ", name )
   print ( "Country: ", country )
# Now you can call printinfo function
printinfo( "Sherin", "USA" )
printinfo( "Sherin" )
printinfo( country = "Israel", name = "Mikhael" )

The first printinfo() call denotes how the default value for "country" is overridden when a value is passed by the caller. The second printinfo() call denotes how default value is taken if no corresponding value is passed. It is not mandatory to specify the parameters in the order in which they occur in the function definition. To ensure that correct value is mapped to each input parameter, we can specify the parameters as comma-separated param=value pairs. The third call to printinfo() denotes how this can be done.

Output for the above program will be:

Name: Sherin
Country: USA
Name: Sherin
Country: India
Name: Mikhael
Country: Israel

© 2019 Sam Shepards


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