BUG: .loc operation cannot locate existing index when having single string as index for dataframe ('string',)

[carlonlv]

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Reproducible Example

import pandas as pd
temp = pd.DataFrame({'a': [1,2,3], 'b': False}, index=[('h',), ('v',), ('c',)])
print(('h',) in temp.index) ## This would print True
temp.loc[('h',), 'b'] = True ## This would result in key error

Issue Description

It seems like when having indices looking at ('a',), pandas automatically converts it into string 'a'.

KeyError: "None of [Index(['h'], dtype='object')] are in the [index]"

Expected Behavior

temp.loc[('h',)] operation should be successful

Installed Versions

INSTALLED VERSIONS ------------------ commit : bdc79c1 python : 3.11.5.final.0 python-bits : 64 OS : Linux OS-release : 6.5.0-1015-azure Version : #15~22.04.1-Ubuntu SMP Tue Feb 13 01:15:12 UTC 2024 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : C.UTF-8 LOCALE : en_US.UTF-8

pandas : 2.2.1
numpy : 1.26.3
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 68.2.2
pip : 23.3.1
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.3
IPython : 8.20.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
bottleneck : 1.3.7
dataframe-api-compat : None
fastparquet : None
fsspec : 2023.12.2
gcsfs : None
matplotlib : 3.8.3
numba : None
numexpr : 2.8.7
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : 0.19.0
tzdata : 2023.3
qtpy : None
pyqt5 : None

[carlonlv]

Note that the same operation with tuples with length >= 2 works as expected.

Also in the above example, if I do temp.loc[temp.index == ('h',), 'b'] = True, it also works fine

[mbarki-mohamed]

Please have a look at the following answer on StackOverflow; (it's an old one but still relevant)
https://stackoverflow.com/questions/40186361/pandas-dataframe-with-tuple-of-strings-as-index

It will work if you pass the tuple inside a list as argument to the loc function (explanation in the link)
temp.loc[('h',), 'b'] = True # error
to
temp.loc[[('h',)], 'b'] = True # works fine

Hope this helps !

[carlonlv]

Thanks for the clarification. I was simply using the dataframe as dictionary. I guess this is one of the "gotcha" moment.

[rhshadrach]

Thanks for the report. pandas uses tuples to signify values in a MultiIndex, and this is the reason why your lookup fails. One idea is to treat non-MulitIndexes differently here, allowing for lookups with tuples, whereas supplying tuples when the index is a MultiIndex would interpret them as levels. Perhaps this has some bad implications though, further investigations are welcome!