Generators and nested functions

[Edward C. Jones]

The following code would be a simple way for doing a useful thing except 
that it doesn't work. Why not? Any easy fixes?

from __future__ import generators
import os

def walk_dirs(startdir):
     def visit(arg, dirname, names):
	yield dirname

     os.path.walk(startdir, visit, None)

if __name__ == '__main__':
     for dir in walk_dirs('/some/small/directory'):
	print dir