Broken in 1.10.0: Cannot extract struct values within list

[aersam]

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

print(
    pl.DataFrame(
        [
            {
                "a": [
                    {
                        "b": 1.0,
                        "c": 4,
                    }
                ]
            }
        ],
        {
            "a": pl.List(
                pl.Struct(
                    {
                        "b": pl.Float64(),
                        "c": pl.Float64(),
                    }
                )
            ),
        },
    ).select(
        pl.col("a").list.eval(
            pl.element().struct.field("b"),
        )
    )
)

Log output

Traceback (most recent call last):
  File "C:\Projects\BMS_Github\deltalake2db\repo.py", line 25, in <module>
    ).select(
      ^^^^^^^
  File "C:\Projects\BMS_Github\deltalake2db\.venv\Lib\site-packages\polars\dataframe\frame.py", line 9021, in select
    return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Projects\BMS_Github\deltalake2db\.venv\Lib\site-packages\polars\lazyframe\frame.py", line 2055, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
polars.exceptions.ColumnNotFoundError

Issue description

This should work as it did up to 1.9.0

Expected behavior

Print the dataframe

Installed versions

--------Version info---------
Polars:              1.10.0
Index type:          UInt32
Platform:            Windows-10-10.0.19045-SP0
Python:              3.11.1 (tags/v3.11.1:a7a450f, Dec  6 2022, 19:58:39) [MSC v.1934 64 bit (AMD64)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            0.18.1
fastexcel            <not installed>
fsspec               2024.10.0
gevent               <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                2.1.2
openpyxl             <not installed>
pandas               2.2.3
pyarrow              17.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[cmdlineluser]

It seems there is now some general breakage due to how pl.element() is currently implemented.

>>> print(pl.element())
col("")

#19317

[aersam]

If pl.element is completely broken, I really hope for a fast bug fix... This could break lots of stuff

[ritchie46]

Can we get a bisect here?

[cmdlineluser]

@ritchie46 It seemed to be #19195 to me.

(I posted some extra details in #19317)

pl.element() still works, it is not completely broken:

>>> pl.Series([[1, 2]]).list.eval(pl.element() + 1)
shape: (1,)
Series: '' [list[i64]]
[
	[2, 3]
]

It just seems that there a few different ways to hit the col("") lookup behaviour.

[adamreeve]

We've also hit this regression and I can confirm it is caused by #19195, this works with the parent commit (1cc2cb8) but fails with 251e171.