How does os.walk work?

[gregpinero at gmail.com]

In the example from help(os.walk) it lists this:

    from os.path import join, getsize
    for root, dirs, files in walk('python/Lib/email'):
        print root, "consumes",
        print sum([getsize(join(root, name)) for name in files]),
        print "bytes in", len(files), "non-directory files"
        if 'CVS' in dirs:
            dirs.remove('CVS')  # don't visit CVS directories

What I'm wondering is how does the "dirs.remove('CVS')" line prevent
os.walk from visiting that directory?  how does the walk function know
what you do to the dirs variable?  I tried looking at the code in
os.py but it wasn't clear to me there either.

Thanks,

Greg