The concept known as "functional abstraction" is a cornerstone of computer science. A thorough understanding of how to read, write, and use functions properly is a prerequisite both for writing good software and for understanding concepts in computer science.
A function is a set of instructions grouped together which
altogether perform some useful task—particularly a task that
will need to be done in several different places in a program. For example
programs that need to do some geometry such as our physics programs
often need to compute the sine of various angles. Python provides a
function called sin() for this purpose. The sequence of
instructions which performs the sine function accurately on a typical
computer is surprisingly long and complicated. Rather than cutting
and pasting the instructions to compute sine wherever you need to compute the sine of an
angle, you can simply use the sin() function.
a = sin(pi)This call to the
sin() function executes all of the
instructions needed to compute the sine of π
and assigns that value to the variable a. The
pi in the parentheses is called the argument to the
function. The value it computes is called its "return value". You can
imagine that the computer substitutes the return value in place of the
function call so in this example sin(pi) gets replaced
with '0' so the statement becomes a =
0. Similarly, the statement print sin(pi) would just print out '0'.
Functions may have several arguments separated by commas:
answer = ultimate_answer( life, universe, everything ) print answer
It is also possible to have a function that has no arguments if that function needs no information to perform its task. However when you call any function you must always supply the parentheses for python to recognize you are trying to call a function:
weather = random()As described above, Python functions may also return more than one thing at a time. The following function returns two values which are assigned to 'x' and 'y' respectively:
x,y = displacement(t,x0,v,a)
In mathematics, a function is a computation that evaluates to a single numerical value which usually depends on the value of one or more numerical arguments. In Python, a function is not restricted to returning a numerical value. A Python function returns a Python "object" which can be a number, a string of characters, or more complex objects. Python functions may return more than one value or nothing at all. In computer science, functions which don't return anything are known as "procedures".
Well it wouldn't be good for anything at all except that functions can have "side effects". You could call a function which prints out something on the screen but doesn't return a value. Printing a message was a useful side effect. Other side effects might include displaying graphics, opening and closing files, or modifying the values of the function's arguments.
sin(angle) than to have all of the complex code to do it
copied everywhere in my program I need it. Imagine if every time a
recipe called for stirring the ingredients it said, "pick up spoon,
lower spoon into bowl, move spoon in circular motion, repeat,
repeat..."Groups of related functions which are usually expected to be used
together are called a "library". For example the math
library includes the most commonly used math functions to compute
things like sine, cosine, and square root. The corresponding math
library functions are sin(), cos(), and
sqrt() respectively. Libraries are typically designed for
use by programs written in particular languages. Libraries written for
use with Python are called "modules" which exist on your hard disk in
various forms. They may be Python programs you can read, or they may
be "compiled" into an "executable" format that can only be read by the
computer but which runs much faster. On Windows these appear as ".dll"
files.
Before you can use a function in your program you must tell the Python interpreter where to find the code which must be executed when you call that function. This code is in a module as described above. You tell Python you want to use the functions in a library by "importing" its module like this:
from math import *This statement says, "import all of the functions in the
math library. If you only want to use one or two
functions out of a library, you can say that like this:
from math import sin, cos, tan
If you get that error, first make sure you spelled the module name right! Python searches for modules in a predefined list of directories for a file matching the name of the module being imported but with a '.py' or '.pyc' extension. First it looks in the current directory, then in the list of directories in an environment variable called PYTHONPATH. The method to examine and modify environment variables varies from one operating system to another and is beyond the scope of this FAQ. There is an explanation in the Modules chapter of the Python Tutorial.
The first place to explore is the Python Library Reference which documents the functions included with a standard distribution of Python. One special group of functions are the "builtin" functions which are always available without being imported. Global Modules Index.
A popular tool for creating wrappers for use by various languages including Python is called SWIG, the "simplified wrapper and interface generator".