Defining a function

def <function_name>(<parameters>):

<statements to perform when the function is called>

The def keyword tells Python that you wish to define a function. On the same line the function interface is defined, which is its name and a comma-separated list of parameters, followed by a colon. If the function has no parameters, then there is nothing inside the parentheses; however, the parentheses themselves must still be included.

The line following this must be indented, and all subsequent lines which are similarly indented belong to the body of the function. This is the code that will be executed when you come to use the function. Recall that Python relies entirely on indentation to understand code structure.

The body of the function can contain any valid Python code, including loops and conditional statements. It can even contain more function definitions. The function body ends when there is a line of code which is not indented compared to the line beginning def (or when the file ends).

Parameters are a special sort of variable. They can be used inside the function exactly like a variable, but they cannot be used outside of the function. The value of a parameter is defined when the function is called.