Victor, great work on pushing this out, especially with the "modifying the directories" fix. (And thanks Serhiy for pushing on correctness here.)

Couple of comments/questions about your new os.walk() implementation.

1) The new implementation is more complex. Of course, most of this is necessary due to the topdown directory issue. However, one thing I'm not sure about is why you create scandir_it manually and use a while True loop, adding complexity and making it require two versions of the error handling. Why not a simple "for entry in scandir(top): ..." with a try/except OSError around the whole loop? I could well be missing something here though.

2) In this commit http://bugs.python.org/review/23605/diff/14181/Lib/os.py -- which is not the final one, I don't quite understand the catch_oserror thing. Presumably this turned out to be unnecessary, as it's not in the final version?

3) Really minor thing: in one of the comments, you misspell "symbolik". Should be "symbolic".