In [1]: import pandas as pd In [2]: df = pd.DataFrame({"A": [0, 0, 1, None], "B": [1, 2, 3, None]}) In [3]: gb = df.groupby("A", dropna=True) 

In [4]: gb['B'].transform(len) In [5]: gb[['B']].transform(len) In [6]: gb.transform(len) 

--------------------------------------------------------------------------- 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

Out[9]: B 0 2 1 2 2 1