help needed on generator

[Gonçalo Rodrigues]

Hi,

I've coded this generator to traverse a directory tree. 

#Import generators.
from __future__ import generators

#Import modules.
import os, sys

def true(*args, **kwargs):
    """The identically true function."""
    return 1

def dirtree(root, predicate = true):
    """Visit a directory tree, using a generator.

    A predicate can be provided to trim the directories visited."""

    #Yield root dir.
    if predicate(root):
        yield root

    #Recurse into subdirectories.
    for dirname in [dir for dir in os.listdir(root) if
os.path.isdir(dir)]:
        if predicate(dirname):
            dirtree(os.path.join(root, dirname), predicate)


if __name__ == '__main__':
    try:
        root = sys.argv[1]
    except IndexError:
        root = os.getcwd()

    for dir in dirtree(root):
        print dir

When I feed "C:" to the sucker (run as script) only "C:" gets printed.
Can anyone point out to me what I am doing wrong?

P.S: I want to keep the order: first yield the directory then go and
visit the childs.

Thanks in advance and best regards,
Gonçalo Rodrigues