+def get_top_dir(root, path):
+ """
+ Return the directory in path that is a subdirectory of root.
+ e.g.
+
+ >>> print get_top_dir('/a/b/c', '/a/b/c/d/e/f')
+ d
+ >>> print get_top_dir('/a/b/c/', '/a/b/c/d/e/f')
+ d
+ >>> print get_top_dir('/a/b/c', '/g/e/f')
+ None
+ >>> print get_top_dir('/a/b/c', '/a/b/c')
+ <BLANKLINE>
+ """
+ if path.startswith(root):
+ subpath = path[len(root):]
+ if subpath.startswith('/'):
+ subpath = subpath[1:]
+ return subpath.split(os.path.sep)[0]
+ else:
+ return None
+