BUG: calling at[] on a Series with a single-level MultiIndex returns a Series, not a scalar

[bpteague]

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

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

• [NO] (optional) I have confirmed this bug exists on the master branch of pandas.

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Code Sample, a copy-pastable example

idx1 = pd.MultiIndex.from_tuples([(False, 1.0), 
                                  (False, 10.0),
                                  (True, 1.0), 
                                  (True, 10.0)])
s1 = pd.Series((0, 1, 2, 3), index = idx1)
s1.at[(False, 1.0)]   # returns '0', expected output

idx2 = pd.MultiIndex.from_tuples([(False,), (True,)])
s2 = pd.Series((0, 1), index = idx2)
s2.at[False]  # returns a pandas.Series with one element

Problem description

As per the user guide, I expect at[] to return a scalar. However, when I index a Series with a MultiIndex with only one level, at[] returns a Series with a single element. A Series with a multi-level MultiIndex works as expected.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : db08276
python : 3.6.12.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-54-generic
Version : #60-Ubuntu SMP Fri Nov 6 10:37:59 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.3
numpy : 1.19.2
pytz : 2020.1
dateutil : 2.8.1
pip : 20.2.4
setuptools : 50.3.1.post20201107
Cython : None
pytest : None
hypothesis : None
sphinx : 3.2.1
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.16.1
pandas_datareader: None
bs4 : None
bottleneck : 1.3.2
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.3.2
numexpr : 2.7.1
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.5.2
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[bpteague]

I wonder if this is related to #29749?

[phofl]

Hi,

thanks for your report. #29749 did not fix this.