def walk(self, top, topdown=True, ignore_file_handler=None):
        """Directory tree generator.

        See `os.walk` for the docs. Differences:
        - no support for symlinks
        - it could raise exceptions, there is no onerror argument
        """

        tree = self.git_object_by_path(top)
        if tree is None:
            raise IOError(errno.ENOENT, "No such file")

        for x in self._walk(tree, topdown):
            yield x