3.4.1. Function syntax

Functions are defined using this syntax:

def function_name(arg1, arg2):
    operation1()
    operation2()
    operation3()
    return the_result

This reads as “Define the function function_name, which takes arg1, arg2 as inputs. This function will process these inputs and return the_result as an output.

Note

• The inputs of a function are called arguments or parameters. Both terms are used interchangeably.

• The output of a function is called a return value.

Note

There is no minimal or maximal limit in the number of arguments. We could very well have a function with five arguments:

def function_name(arg1, arg2, arg3, arg4, arg5):

Here is an example of a function that prints its arguments’ values and their sum to the console and that returns nothing:

def print_sum(arg1, arg2):
    print("Hi!")
    print(f"The first variable is {arg1}.")
    print(f"The second variable is {arg2}.")
    print(f"The sum of both variables is {arg1 + arg2}.")

The first line of the function definition is called the function signature. It defines how to use the function. By reading the signature of print_sum, we know that the function is named print_sum and that it expects two arguments.

Note the colon : that terminates the function signature. A colon indicates that the following indented lines are a code block, which is simply a series of lines that belong to a same group. In the case of a function, this code block is the function implementation. These are the lines to be executed when the function is called. In Python, code blocks are delimited by their indentation. Every line of a code block must be indented with the same number of spaces.

Tip

Since indentation is so important in Python, practically all Python editors include keyboard shortcuts to indent or outdent code blocks. In Spyder, select a group of lines, then press Tab to indent these lines altogether, or Shift+Tab to outdent.

Once a function is defined (using the def keyword), it can be called as many times as needed:

print_sum(1, 4)
print_sum(3, 10)

Hi!
The first variable is 1.
The second variable is 4.
The sum of both variables is 5.
Hi!
The first variable is 3.
The second variable is 10.
The sum of both variables is 13.

Good practice: Function naming

Usually, function names start with an active verb that tells what action is performed. Usually in Python, function names are written in lower_case style, with words separated by underscores.