Called when at a leaf value. Should yield a value.

def _AtLeaf(self, attr_value):
    """Called when at a leaf value. Should yield a value."""
    if isinstance(attr_value, collections.Mapping):
      # If the result is a dict, return each key/value pair as a new dict.
      for k, v in iteritems(attr_value):
        yield {k: v}
    else:
      yield attr_value