fix: prevent panic with arg_sort_by

[eitsupi]

From pola-rs/r-polars#929

Correct panic when multiple column names are specified as the first argument of arg_sort_by.

In Python, current release version:

>>> import polars as pl
>>> df = pl.DataFrame(
...     {
...         "a": [0, 1, 1, 0],
...         "b": [3, 2, 3, 2],
...     }
... )
>>> df.select(pl.arg_sort_by(pl.col("a")))
shape: (4, 1)
┌─────┐
│ a   │
│ --- │
│ u32 │
╞═════╡
│ 0   │
│ 3   │
│ 1   │
│ 2   │
└─────┘
>>> df.select(pl.arg_sort_by(pl.col("a", "b")))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/workspaces/polars/py-polars/polars/functions/lazy.py", line 1601, in arg_sort_by
    return wrap_expr(plr.arg_sort_by(exprs, descending))
polars.exceptions.ComputeError: this expression may produce multiple output names
>>> df.select(pl.arg_sort_by("*"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/workspaces/polars/py-polars/polars/functions/lazy.py", line 1601, in arg_sort_by
    return wrap_expr(plr.arg_sort_by(exprs, descending))
polars.exceptions.ComputeError: cannot determine output column without a context for this expression

[codecov]

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[stinodego]

This shouldn't panic, but I think it should just work rather than throw an error.

Also, can you add the example you gave in the PR description as a test in the Python test suite?

[eitsupi]

This shouldn't panic, but I think it should just work rather than throw an error.

My understanding is that if something like Expr::Wildcard is passed as a column name, it cannot be reused as a column name in the result, so an error will occur.

>>> import polars as pl
>>> pl.arg_sort_by("*")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/workspaces/polars/py-polars/polars/functions/lazy.py", line 1601, in arg_sort_by
    return wrap_expr(plr.arg_sort_by(exprs, descending))
polars.exceptions.ComputeError: cannot determine output column without a context for this expression

[ritchie46]

Can you try keep_name here? That might work.

[eitsupi]

I tried name().keep(), but `keep`, `suffix`, `prefix` should be last expression error occurred in Python tests.

[codspeed-hq]

CodSpeed Performance Report

Merging #15247 will not alter performance

_{Comparing eitsupi:fix-panic-arg_sort_by (04fa3cf) with main (42d3697)}

Summary

✅ 21 untouched benchmarks