BUG: df.assign no longer works with multilevel columns

[Zendaug]

Pandas version checks

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

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import numpy as np

# Creating a DataFrame with multilevel columns
arrays = [['A', 'A', 'B', 'B'], ['one', 'two', 'one', 'two']]
tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
df = pd.DataFrame(np.random.randn(3, 4), columns=index)

# Try to create a new column using "assign"
df = df.assign(**{('C', 'one'): [1, 2, 3]})

Issue Description

The final line reports the error: TypeError: keywords must be strings

Expected Behavior

It should create a new column ("C", 'one') with 1,2,3 in it.

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.12.3
python-bits : 64
OS : Windows
OS-release : 11
Version : 10.0.26100
machine : AMD64
processor : Intel64 Family 6 Model 154 Stepping 4, GenuineIntel
byteorder : little
LC_ALL : None
LANG : en
LOCALE : English_Australia.1252

pandas : 2.2.3
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
pip : 25.0.1
Cython : None
sphinx : 7.3.7
IPython : 8.30.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
blosc : None
bottleneck : 1.4.2
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.6
lxml.etree : 5.3.0
matplotlib : 3.10.0
numba : None
numexpr : 2.10.1
odfpy : None
openpyxl : 3.1.5
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 19.0.0
pyreadstat : 1.2.7
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.15.2
sqlalchemy : None
tables : None
tabulate : 0.9.0
xarray : None
xlrd : None
xlsxwriter : 3.1.1
zstandard : None
tzdata : 2023.3
qtpy : 2.4.1
pyqt5 : None

[asishm]

This is a python limitation. You're using tuple unpacking to pass in arguments to a function. Function argument names have to be valid variable names. https://stackoverflow.com/questions/65392503/keyword-error-generated-when-passing-a-dictionary-to-a-function-with-tuples-as-t

[arthurlw]

Thanks for raising this! The assign function accepts **kwargs which only allows string keys, so tuple-based MultiIndex labels can’t be passed this way.

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.assign.html

Technically speaking this isn't a pandas bug, but maybe pandas could consider supporting tuple keys?

[asishm]

pandas could improve the error message

this is not possible (with the current signature), the error is thrown directly by the python interpreter.

In [1]: def foo(**kwargs):
    ...:     print(kwargs)
    ...:

In [2]: foo(**{('C', 'one'): 2})
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 1
----> 1 foo(**{('C', 'one'): 2})

TypeError: keywords must be strings

[rhshadrach]

Agreed @asishm - nothing can be done here. Closing.