BUG: idxmin() fails for nullable integer data type (Int64)

[cklb]

• 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

import pandas as pd

s = pd.Series([1, 2, 3, 4], dtype="Int64")
s.idxmax()

Problem description

Currently, the idxmax call fails with

  File "~lib/python3.8/site-packages/pandas/core/series.py", line 2110, in idxmax
    i = nanops.nanargmax(com.values_from_object(self), skipna=skipna)
  File "~lib/python3.8/site-packages/pandas/core/nanops.py", line 64, in _f
    raise TypeError(
TypeError: reduction operation 'argmax' not allowed for this dtype

being raised. This may be a duplicate of #32749, although the focus there seems to be generating the exception I already encountered.

Expected Output

Should be 3, I guess.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.8.2.final.0
python-bits : 64
OS : Linux
OS-release : 5.5.13-arch2-1
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : de_DE.UTF-8
LOCALE : de_DE.UTF-8

pandas : 1.0.3
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 41.2.0
Cython : None
pytest : None
hypothesis : None
sphinx : 2.4.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.13.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : 3.2.0
numexpr : None
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : None
tables : None
tabulate : 0.8.6
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : None
numba : None
None

[dsaxton]

Thanks, I would say this is a distinct issue. The error on master is a little different:

[ins] In [1]: ser = pd.Series([1, 2, 3], dtype="Int64")                                                                                                                                                      

[ins] In [2]: ser.idxmax()                                                                                                                                                                                   
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-b02670299a79> in <module>
----> 1 ser.idxmax()

~/pandas/pandas/core/series.py in idxmax(self, axis, skipna, *args, **kwargs)
   2077         """
   2078         skipna = nv.validate_argmax_with_skipna(skipna, args, kwargs)
-> 2079         i = nanops.nanargmax(self._values, skipna=skipna)
   2080         if i == -1:
   2081             return np.nan

~/pandas/pandas/core/nanops.py in _f(*args, **kwargs)
     69             try:
     70                 with np.errstate(invalid="ignore"):
---> 71                     return f(*args, **kwargs)
     72             except ValueError as e:
     73                 # we want to transform an object array

~/pandas/pandas/core/nanops.py in nanargmax(values, axis, skipna, mask)
    906         values, True, fill_value_typ="-inf", mask=mask
    907     )
--> 908     result = values.argmax(axis)
    909     result = _maybe_arg_null_out(result, axis, mask, skipna)
    910     return result

AttributeError: 'IntegerArray' object has no attribute 'argmax'

[jorisvandenbossche]

This is related to #27801