A Python walker that can ignore directories

I need a file system walker that I could instruct to ignore traversing directories that I want to leave untouched, including all subdirectories below that branch. The os.walk and os.path.walk just don't do it.

So I made this home-roles walker function:

import os
from os.path import join, isdir, islink, isfile

def mywalk(top, topdown=True, onerror=None, ignore_list=('.ignore',)):
    try:
        # Note that listdir and error are globals in this module due
        # to earlier import-*.
        names = os.listdir(top)
    except Exception, err:
        if onerror is not None:
            onerror(err)
        return
    if len([1 for x in names if x in ignore_list]):
        return 
    dirs, nondirs = [], []
    for name in names:
        if isdir(join(top, name)):
            dirs.append(name)
        else:
            nondirs.append(name)

    if topdown:
        yield top, dirs, nondirs
    for name in dirs:
        path = join(top, name)
        if not islink(path): 
            for x in mywalk(path, topdown, onerror, ignore_list):
                yield x
    if not topdown:
        yield top, dirs, nondirs