Help me understand this iterator

[LaundroMat]

Hi,

I've found this script over at effbot
(http://effbot.org/librarybook/os-path.htm), and I can't get my head
around its inner workings. Here's the script:

import os

class DirectoryWalker:
    # a forward iterator that traverses a directory tree

    def __init__(self, directory):
        self.stack = [directory]
        self.files = []
        self.index = 0

    def __getitem__(self, index):
        while 1:
            try:
                file = self.files[self.index]
                self.index = self.index + 1
            except IndexError:
                # pop next directory from stack
                self.directory = self.stack.pop()
                self.files = os.listdir(self.directory)
                self.index = 0
            else:
                # got a filename
                fullname = os.path.join(self.directory, file)
                if os.path.isdir(fullname) and not
os.path.islink(fullname):
                    self.stack.append(fullname)
                return fullname

for file in DirectoryWalker("."):
    print file

Now, if I look at this script step by step, I don't understand:
- what is being iterated over (what is being called by "file in
DirectoryWalker()"?);
- where it gets the "index" value from;
- where the "while 1:"-loop is quitted.

Thanks in advance,

Mathieu