Recursively list files in a directory

"""
Usage:

>>> list_files('/tmp')
['/tmp/one', '/tmp/two', ... ]
"""
import os

def list_files(directory, files=[]):
    for item in os.listdir(directory):
        _item = os.path.join(directory, item)
        if os.path.isfile(_item):
            files.append(_item)
        else:
            list_files(_item, files)
    return files