Python Essential Reference, Second Edition

Functions

Functions are defined with the def statement:

def add(x,y): 
    return x+y 

Invoke the function by writing the function name followed by a tuple of function arguments, such as a = add(3,4). The order and number of arguments must match those given in the function definition. If a mismatch exists, a TypeError exception is raised.

By assigning values, you can attach default arguments to function parameters:

def foo(x,y,z = 42): 

When a function defines a parameter with a default value, that parameter and all the parameters that follow are optional. If values are not assigned to all the optional parameters in the function definition, a SyntaxError exception is raised.

Default parameter values are always set to the objects that were supplied ...