During the discussion about adding a chowntree function to shutil (http://mail.python.org/pipermail/python-dev/2011-May/111661.html and ), Victor suggested this:

> I don't like the idea of a recursive flag. I would prefer a "map-like" function to "apply" a
> function on all files of a directory. Something like shutil.apply_recursive(shutil.chown)...
> ... maybe with options to choose between deep-first search and breadth-first search, filter
> (filenames, file size, files only, directories only, other attributes?), directory before
> files (may be need for chmod(0o000)), etc.

Such a function removes the need for copytee, rmtree, dosomethingelsetree, etc.  When I first read this feature request I thought it was the same idea, but after reading it again I see that you propose a function that walks and returns, not a function that walks and applies a callback.  Both use cases are important, so an apply_tree function could be implemented on top of your filter_walk (I’ll open another report if we reach agreement on the idea of these two functions working together).