BUG: Regression on SeriesGrouper using Timestamp index with pandas 1.3.0

[philpep]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

import pandas as pd
import numpy as np


def agg(series):
    if series.isna().values.all():
        return None
    return np.sum(series)


df = pd.DataFrame([1.0], index=[pd.Timestamp("2018-01-16 00:00:00+00:00")])
df.groupby(lambda x: 1).agg(agg)

Problem description

This raise with 1.3.0 and succeeded with 1.2.5

Traceback (most recent call last):
  File "/tmp/t.py", line 12, in <module>
    df.groupby(lambda x: 1).agg(agg)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 1010, in aggregate
    result = gba.agg()
  File "/home/phil/src/pandas/pandas/core/apply.py", line 164, in agg
    return self.agg_list_like()
  File "/home/phil/src/pandas/pandas/core/apply.py", line 355, in agg_list_like
    new_res = colg.aggregate(arg)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 249, in aggregate
    ret = self._aggregate_multiple_funcs(func)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 303, in _aggregate_multiple_funcs                                                                                        
    results[key] = self.aggregate(func)
  File "/home/phil/src/pandas/pandas/core/groupby/generic.py", line 265, in aggregate
    return self._python_agg_general(func, *args, **kwargs)
  File "/home/phil/src/pandas/pandas/core/groupby/groupby.py", line 1310, in _python_agg_general                                                                                             
    result = self.grouper.agg_series(obj, f)
  File "/home/phil/src/pandas/pandas/core/groupby/ops.py", line 1028, in agg_series
    result = self._aggregate_series_fast(obj, func)
  File "/home/phil/src/pandas/pandas/core/groupby/ops.py", line 1053, in _aggregate_series_fast                                                                                              
    result, _ = sgrouper.get_result()
  File "pandas/_libs/reduction.pyx", line 281, in pandas._libs.reduction.SeriesGrouper.get_result                                                                                            
    cached_series, cached_index, islider, vslider)
  File "pandas/_libs/reduction.pyx", line 88, in pandas._libs.reduction._BaseGrouper._apply_to_group                                                                                         
    res = self.f(cached_series)
  File "/home/phil/src/pandas/pandas/core/groupby/groupby.py", line 1296, in <lambda>
    f = lambda x: func(x, *args, **kwargs)
  File "/tmp/t.py", line 6, in agg
    if series.isna().values.all():
  File "/home/phil/src/pandas/pandas/core/series.py", line 5161, in isna
    return generic.NDFrame.isna(self)
  File "/home/phil/src/pandas/pandas/core/generic.py", line 7145, in isna
    return isna(self).__finalize__(self, method="isna")
  File "/home/phil/src/pandas/pandas/core/dtypes/missing.py", line 138, in isna
    return _isna(obj)
  File "/home/phil/src/pandas/pandas/core/dtypes/missing.py", line 177, in _isna
    result, index=obj.index, name=obj.name, copy=False
  File "/home/phil/src/pandas/pandas/core/series.py", line 430, in __init__
    com.require_length_match(data, index)
  File "/home/phil/src/pandas/pandas/core/common.py", line 528, in require_length_match
    "Length of values "
ValueError: Length of values (1) does not match length of index (0)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : c8b79e1bd30fb4d36e5c1f894757d59c57837528
python : 3.7.3.final.0
python-bits : 64
OS : Linux
OS-release : 4.19.0-17-amd64
Version : #1 SMP Debian 4.19.194-2 (2021-06-21)
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.3.0+4.gc8b79e1bd3
numpy : 1.21.0
pytz : 2021.1
dateutil : 2.8.1
pip : 21.1.3
setuptools : 57.1.0
Cython : 0.29.23
pytest : 6.2.4
hypothesis : 6.14.1
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[simonjayhawkins]

This raise with 1.3.0 and succeeded with 1.2.5

first bad commit: [c355ed1] REF: simplify libreduction (#41262)

cc @jbrockmendel

[simonjayhawkins]

@jbrockmendel i've closed #42391 as stale.