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Function Basics

The def statement is the signal to Python that you're about to define a function. You follow the keyword def with the name of the function you're defining, and a parenthesized list of the arguments which are to be passed to a function. If your function does not accept any arguments, then simply use an empty set of parentheses. A colon (:) follows the parenthesized list. The remainder of the function needs to be indented similarly to the body of a loop (Section 6.2). If the very next line of your function definition is a quoted string, it's stored along with your function to provide documentation about the function; such a string is called a docstring. The function body follows the def line and the optional docstring. The function will return control to the calling environment when it encounters a return statement, or when it reaches the end of the indented function body, whichever comes first. If an expression appears after the return statement and on the same line, the value of that expression is returned through a call to the function. If no expression appears on the same line as the return statement, or the function does not contain a return statement, a call to such a function will return the value of None.

You can access the docstring of a function by refering to the __doc__ attribute, or by passing the name of the function to the help function when running python interactively. In addition, some python-aware editors will display the docstring as a tool tip when you move your cursor over the function's name in your program.

Here's a illustration of a function; the function merge combines two lists, adding items from the second list only if they do not appear in the first.

def merge(list1,list2):
'''merge(list1,list2) returns a list consisting of the original list1
along with any elements of list2 which were not already in list 1'''
    newlist = list1[:]
    for i in list2:
        if i not in newlist:
    return newlist
Notice that, inside the function, newlist was created from list1 using the techniques discussed in Section 6.1, so that the first list passed to merge would not be modified by the function. The return statement is required, since the goal of the function is to provide a new list which combines the elements of the two lists passed to the function.

To call a function, you refer to its name with a parenthesized list of arguments; if the function takes no arguments, you follow the function name with a set of empty parentheses, so that Python won't confuse the function call with a reference to an ordinary variable. Arguments to functions behave much the way that assignments do (See Section 6.1): modifying a scalar or immutable object which has been passed through the argument list of a function will not modify the object itself, but modifying the elements of a mutable object (like a list or dictionary) passed to a function will actually change those elements in the calling environment. The following program shows how the merge function could be called. For now, we'll assume that the definition of was typed in interactively before the example, or, if the program was in a file, the function definition appeared earlier in the same file as the example. Later we'll see how you can use the import statement to access function definitions from other files, without repeatedly entering the function definition.

>>> one = [7,12,19,44,32]
>>> two = [8,12,19,31,44,66]
>>> print merge(one,two)
[7, 12, 19, 44, 32, 8, 31, 66]
>>> print one
[7, 12, 19, 44, 32]

next up previous contents
Next: Named Arguments and Default Up: Functions Previous: Scoping: How Python finds   Contents
Phil Spector 2003-11-12