Pandas DataFrames with a multiindex fails to be converted to a Polars DataFrame

[mwouts]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import pandas as pd
import polars as pl

df = pd.DataFrame(
    range(4),
    columns=['A'],
    index=pd.MultiIndex.from_product((["C", "D"], [3, 4])),
)

pl.from_pandas(df)

Log output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/marc/miniconda3/envs/itables/lib/python3.11/site-packages/polars/convert/general.py", line 570, in from_pandas
    pandas_to_pydf(
  File "/home/marc/miniconda3/envs/itables/lib/python3.11/site-packages/polars/_utils/construction/dataframe.py", line 1078, in pandas_to_pydf
    _check_pandas_columns(data)
  File "/home/marc/miniconda3/envs/itables/lib/python3.11/site-packages/polars/_utils/construction/dataframe.py", line 1059, in _check_pandas_columns
    raise ValueError(msg)
ValueError: Pandas dataframe contains non-unique indices and/or column names. Polars dataframes require unique string names for columns.

Issue description

The code above passes with polars==1.1.0, but not with 1.2.0 to 1.4.1

Expected behavior

I expect to be able to call pl.from_pandas(df) on a dataframe with a multiindex

Installed versions

>>> pl.show_versions()
--------Version info---------
Polars:               1.4.1
Index type:           UInt32
Platform:             Linux-6.5.0-35-generic-x86_64-with-glibc2.35
Python:               3.11.9 (main, Apr 19 2024, 16:48:06) [GCC 11.2.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           3.8.4
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.2
pyarrow:              14.0.2
pydantic:             <not installed>
pyiceberg:            <not installed>
sqlalchemy:           2.0.30
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[ritchie46]

@MarcoGorelli could you take a look at this one? Might be related to our own from_pandas logic?

[MarcoGorelli]

Hey - looks like this was caused by #17628

[MarcoGorelli]

The index in this case has a name of None for both levels:

In [4]: df.index.names
Out[4]: FrozenList([None, None])

So, if include_index=True had been passed, I'd agree with the error message

But, given that the default is include_index=False, I think this should pass, and so this is a bug

[mwouts]

Thank you all for the super fast resolution! Much appreciated!