What does a function do?

[Remco Gerlich]

alki <webmaster at pawntastic.com> wrote in comp.lang.python:
> So a function is just a group of codes? Then how do you use a function? Will
> you please give me an example? Thanks

Let's take a very simple example, a function that adds two numbers, x and y:

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

The function has two "arguments", named x and y, that can be used inside the
function. The "return" statement tells it to return the sum of x and y.

Now if we do

sum = add(3,4)
print sum

later, that will print 7. "add(3,4)" calls the function, with the values x=3
and y=4 filled in. Therefore it returns 7, and that is called sum.

A function can do a lot more, and use other functions in the same way.

This function checks if a certain word occurs on a webpage (either in the
html or the text itself), and returns 1 if it does, 0 if it doesn't:

def find_word(url, word):
    import urllib, string
    page = urllib.URLopener().open(url).read()
    if string.find(page, word) != -1:
       return 1
    else:
       return 0

This function uses a class from urllib, calls a few functions on it (they
belong to the class, they're "methods", that's why you get the dot notation),
and the function "string.find" (it's the function named "find", inside the
"string" module) to easily get the work done. If find returns something
other than -1, the word is on the page.

The modules are full of already defined functions, and you probably already
use some of them as well - if you have a list l, len(l) gives its length;
and len() is the function that does that.

-- 
Remco Gerlich