pl.exclude(pl.first()) still isn't working

[Wainberg]

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 polars as pl
>>> pl.exclude(pl.first())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../python3.12/site-packages/polars/functions/lazy.py", line 1292, in exclude
    return exclude(columns, *more_columns)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../python3.12/site-packages/polars/selectors.py", line 1323, in exclude
    return ~_combine_as_selector(columns, *more_columns)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../python3.12/site-packages/polars/selectors.py", line 228, in _combine_as_selector
    raise TypeError(
TypeError: invalid input for `exclude`

Expected one or more `str`, `DataType` or selector; found <Expr ['nth(0)'] at 0x7F2DF6AB5D00> instead.```

Log output

No response

Issue description

When I suggested supporting expressions and selectors in pl.exclude (#13247), my motivating example was that pl.exclude(pl.first()) didn't work. Unfortunately, it still doesn't work after #13301, because pl.exclude() accepts selectors but not expressions.

Expected behavior

pl.exclude(pl.first()) should be a valid expression or selector that excludes the first column.

Installed versions

--------Version info---------
Polars:               0.20.3
Index type:           UInt32
Platform:             Linux-3.10.0-1160.102.1.el7.x86_64-x86_64-with-glibc2.17
Python:               3.12.1 | packaged by conda-forge | (main, Dec 23 2023, 08:03:24) [GCC 12.3.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           3.8.2
numpy:                1.26.3
openpyxl:             3.1.2
pandas:               2.1.4
pyarrow:              14.0.2
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             0.8.1
xlsxwriter:           3.1.9

[MarcoGorelli]

Isn't it expected that this raises? You can use cs.first(), which is a column selector. pl.first() selects the first element, not the first column

[Wainberg]

df.select(pl.first()) does select the first column: https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.first.html.

[cmdlineluser]

This function has different behavior depending on the input type:

• None -> Expression to take first column of a context.
• str -> Syntactic sugar for pl.col(column).first()

It seems like perhaps the special cased column selector behaviour needs to go?

As it stands, it basically cannot be combined with anything, it's not just an exclude issue:

df.select(pl.col(pl.first()))
# TypeError: invalid input for `col`

df.drop(pl.first())
# TypeError: argument 'columns': 'Expr' object cannot be converted to 'PyString'

[stinodego]

As Marco pointed out, this should raise, as exclude expects a column name or selector, and pl.first() is an expression. The error message correctly indicates this.

For further discussion on how selectors should work, please refer to #13757

[Wainberg]

Worth mentioning that this also doesn't work for Expr.exclude, but for a different reason:

>>> pl.DataFrame({'a': [1], 'b': [2]}).select(pl.all().exclude(pl.first()))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/michael/miniforge3/lib/python3.12/site-packages/polars/expr/expr.py", line 947, in exclude
    raise TypeError(msg)
TypeError: invalid input for `exclude`

Expected one or more `str` or `DataType`; found <Expr ['nth(0)'] at 0x7F386B584A70> instead.