# Buggy code
def _set_result_index_ordered(
    self, result: "OutputFrameOrSeries"
) -> "OutputFrameOrSeries":
    # set the result index on the passed values object and
    # return the new object, xref 8046

    # the values/counts are repeated according to the group index
    # shortcut if we have an already ordered grouper
    if not self.grouper.is_monotonic:
        index = Index(np.concatenate(self._get_indices(self.grouper.result_index)))
        result.set_axis(index, axis=self.axis, inplace=True)
        result = result.sort_index(axis=self.axis)

    result.set_axis(self.obj._get_axis(self.axis), axis=self.axis, inplace=True)
    return result


# Traceback (ValueError)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-3bae7d67a46f> in <module>
----> 1 gb['B'].transform(len)

/workspaces/pandas-arw2019/pandas/core/groupby/generic.py in transform(self, func, engine, engine_kwargs, *args, **kwargs)
487
488         if not isinstance(func, str):
--> 489             return self._transform_general(
490                 func, *args, engine=engine, engine_kwargs=engine_kwargs, **kwargs
491             )

/workspaces/pandas-arw2019/pandas/core/groupby/generic.py in _transform_general(self, func, engine, engine_kwargs, *args, **kwargs)
556
557         result.name = self._selected_obj.name
--> 558         result.index = self._selected_obj.index
559         return result
560

/workspaces/pandas-arw2019/pandas/core/generic.py in __setattr__(self, name, value)
5167         try:
5168             object.__getattribute__(self, name)
-> 5169             return object.__setattr__(self, name, value)
5170         except AttributeError:
5171             pass

/workspaces/pandas-arw2019/pandas/_libs/properties.pyx in pandas._libs.properties.AxisProperty.__set__()
64
65     def __set__(self, obj, value):
---> 66         obj._set_axis(self.axis, value)

/workspaces/pandas-arw2019/pandas/core/series.py in _set_axis(self, axis, labels, fastpath)
422         if not fastpath:
423             # The ensure_index call above ensures we have an Index object
--> 424             self._mgr.set_axis(axis, labels)
425
426     # ndarray compatibility

/workspaces/pandas-arw2019/pandas/core/internals/managers.py in set_axis(self, axis, new_labels)
214
215         if new_len != old_len:
--> 216             raise ValueError(
217                 f"Length mismatch: Expected axis has {old_len} elements, new "
218                 f"values have {new_len} elements"

ValueError: Length mismatch: Expected axis has 3 elements, new values have 4 elements

# Fixed code
def _set_result_index_ordered(
    self, result: "OutputFrameOrSeries"
) -> "OutputFrameOrSeries":
    # set the result index on the passed values object and
    # return the new object, xref 8046

    if self.grouper.is_monotonic:
        # shortcut if we have an already ordered grouper
        result.set_axis(self.obj._get_axis(self.axis), axis=self.axis, inplace=True)
        return result

    # row order is scrambled => sort the rows by position in original index
    original_positions = Index(
        np.concatenate(self._get_indices(self.grouper.result_index))
    )
    result.set_axis(original_positions, axis=self.axis, inplace=True)
    result = result.sort_index(axis=self.axis)

    dropped_rows = len(result.index) < len(self.obj.index)

    if dropped_rows:
        # get index by slicing original index according to original positions
        # slice drops attrs => use set_axis when no rows were dropped
        sorted_indexer = result.index
        result.index = self._selected_obj.index[sorted_indexer]
    else:
        result.set_axis(self.obj._get_axis(self.axis), axis=self.axis, inplace=True)

    return result