BUG: reindex on empty Series raises ValueError: zero-size array to reduction operation

[TomGoBravo]

• 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

index = pd.MultiIndex.from_tuples([], names=["first", "second"])
s = pd.Series([], index=index, dtype=object)
print(f"s...\n{s}\n")
print(f"reindexed...\n{s.reindex(['a'], level='first')}")

prints

s...
Series([], dtype: object)

and then raises.

Problem description

reindex raises ValueError: zero-size array to reduction operation maximum which has no identity from numpy/core/_methods.py. Traceback:

--------------------------------------------------------------------------- ValueError Traceback (most recent call last) in 3 s = pd.Series(range(len(index)), index=index, dtype=object) 4 print(f"s...\n{s}\n") ----> 5 print(f"reindexed...\n{s.reindex(['a'], level='first')}")

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/pandas/core/series.py in reindex(self, index, **kwargs)
4343 )
4344 def reindex(self, index=None, **kwargs):
-> 4345 return super().reindex(index=index, **kwargs)
4346
4347 def drop(

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/pandas/core/generic.py in reindex(self, *args, **kwargs)
4810 # perform the reindex on the axes
4811 return self._reindex_axes(
-> 4812 axes, level, limit, tolerance, method, fill_value, copy
4813 ).finalize(self, method="reindex")
4814

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/pandas/core/generic.py in _reindex_axes(self, axes, level, limit, tolerance, method, fill_value, copy)
4826 ax = self._get_axis(a)
4827 new_index, indexer = ax.reindex(
-> 4828 labels, level=level, limit=limit, tolerance=tolerance, method=method
4829 )
4830

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/pandas/core/indexes/multi.py in reindex(self, target, method, level, limit, tolerance)
2469 target = ensure_index(target)
2470 target, indexer, _ = self._join_level(
-> 2471 target, level, how="right", return_indexers=True, keep_order=False
2472 )
2473 else:

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/pandas/core/indexes/base.py in _join_level(self, other, level, how, return_indexers, keep_order)
3922 else: # tie out the order with other
3923 if level == 0: # outer most level, take the fast route
-> 3924 ngroups = 1 + new_lev_codes.max()
3925 left_indexer, counts = libalgos.groupsort_indexer(
3926 new_lev_codes, ngroups

~/.pyenv/versions/3.7.9/envs/covid-data-model/lib/python3.7/site-packages/numpy/core/_methods.py in _amax(a, axis, out, keepdims, initial, where)
37 def _amax(a, axis=None, out=None, keepdims=False,
38 initial=_NoValue, where=True):
---> 39 return umr_maximum(a, axis, None, out, keepdims, initial, where)
40
41 def _amin(a, axis=None, out=None, keepdims=False,

ValueError: zero-size array to reduction operation maximum which has no identity

Expected Output

I think it makes sense for s.reindex(['a'], level='first') to return an empty Series, as it does when s is not empty:

index = pd.MultiIndex.from_tuples([('bar', 'one'),('foo', 'two')], names=["first", "second"])
s = pd.Series(range(len(index)), index=index, dtype=object)
print(f"s...\n{s}\n")
print(f"reindexed...\n{s.reindex(['a'], level='first')}")

prints

s...
first  second
bar    one       0
foo    two       1
dtype: object

reindexed...
Series([], dtype: object)

In 1.0.3 I could use df.loc[['label']] to get a slice without an exception when no data was found but that started raising KeyError when I upgraded to 1.2.3. I'm using reindex per https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#reindexing

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f2c8480
python : 3.7.9.final.0
python-bits : 64
OS : Linux
OS-release : 4.19.0-11-amd64
Version : #1 SMP Debian 4.19.146-1 (2020-09-17)
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.2.3
numpy : 1.20.1
pytz : 2020.5
dateutil : 2.8.1
pip : 21.0.1
setuptools : 47.1.0
Cython : None
pytest : 6.2.1
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.19.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : 0.5.0
gcsfs : None
matplotlib : 3.2.1
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.6
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.6.1
sqlalchemy : 1.3.23
tables : None
tabulate : 0.8.7
xarray : 0.16.2
xlrd : 1.2.0
xlwt : None
numba : 0.52.0