3.11 Making your own functions

As we saw in Section 3.9, functions are very useful tools for making your programs more concise and modular.

The libraries provide a useful range of facilities but a programmer will often want or need to write their own functions if, for example, one particular section of a program is to be used several times, or if a section forms a logically complete unit.

Functions must be defined before they are used, so we generally put the definitions at the very top of a program. Here is a very simple example of a function definition that returns the sum of the two numbers it is passed:

    >>> def addnumbers(x, y):
            sum = x + y
            return sum
    
    >>> x = addnumbers(5, 10)
    >>> print x
    15

The structure of the definition is as follows:

The top line must have a def statement: this consists of the word def, the name of the function, followed by parentheses containing the names of the parameters passed as they will be referred to within the function. ^3.12.
Then an indented code block follows. This is what is executed when the function is called, i.e. used.
Finally the return statement. This is the result the function will return to the program that called it. If your function does not return a result but merely executes some statements then it is not required.

If you change a variable within a function that change will not be reflected in the rest of the program. For example:

    >>> def addnumbers(x, y):
            sum = x + y
            x = 1000
            return sum
    
    >>> x = 5
    >>> y = 10
    >>> answer = addnumbers(x, y)
    >>> print x, y, answer
    5 10 15

Note that although the variable x was changed in the function, that change is not reflected outside the function. This is because the function has its own private set of variables. This is done to minimise the risk of subtle errors in your program

If you really want a change to be reflected then return a list of the new values as the result of your function. Lists can then be accessed by offset in the same way as arrays:

    >>> def addnumbers(x, y):
            sum = x + y
            x = 100000
            return [sum, x]
    
    >>> x = 5
    >>> y = 10
    >>> answer = addnumbers(x, y)
    >>> print answer[0]
    15
    >>> print answer[1]
    100000

A fuller discussion of the relationship between variables used in functions and in the main program is discussed in Section 4.4, ``Scope''.

Footnotes

... function.^3.12: These are known as formal parameters (x and y in this case). The actual parameters (5 and 10 in the example) are assigned to the formal parameters when the function is called. Parameters are also often referred to as arguments