Serhiy Storchaka added the comment:
> When followlinks is true, symlinks is not needed.

Hum, it's not easy to understand your code. I guess that the problem
is that a symlink to a directory can become something else (not a
directory or a symlink to a directory).

I noticed this race condition in the new implementation of os.walk(),
but I don't think that the issue is really new. The old implementation
of os.walk() already uses a list of directories. The caller can remove
a directory or replace a directory with something else.

Since the bug is not documented in os.walk(), I chose to not document
it neither. But it would be better to warn users to not do that :-)

@Serhiy: I agree that the new implementation changes the behaviour,
but I don't consider it as as bug. Do you think that it's a bug?