Should pl.concat() accept Expr input?

[etiennebacher]

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

df = pl.DataFrame({"x": [1]})

pl.concat([df, df], how = "foo")
# ValueError: DataFrame `how` must be one of {'vertical', 'vertical_relaxed', 'diagonal', 'diagonal_relaxed', 'horizontal', 'align'}, got 'foo'

df.select(pl.concat([pl.col("x"), pl.col("x")], how = "foo"))
# shape: (2, 1)
# ┌─────┐
# │ x   │
# │ --- │
# │ i64 │
# ╞═════╡
# │ 1   │
# │ 1   │
# └─────┘

Log output

No response

Issue description

Might simply be a documentation issue, but it's unclear to me whether Expr should be accepted input inpl.concat(). The docs of items say:

DataFrames, LazyFrames, or Series to concatenate.

They also don't mention the fact that when the inputs are Expr, how can take any value, even "foo" as in the example above.

Expected behavior

Either an update in the docs saying that Expr are accepted and that in this case how doesn't matter, or forbidding Expr as input.

Installed versions

--------Version info---------
Polars:              1.13.1
Index type:          UInt32
Platform:            Linux-6.8.0-48-generic-x86_64-with-glibc2.39
Python:              3.12.3 (main, Sep 11 2024, 14:17:37) [GCC 13.2.0]
LTS CPU:             False

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

[ritchie46]

No, it's not meant for expressions.

I also don't think it should document what it does not support. It should document what it does. The docs seem pretty clear in that regard.

[eitsupi]

No, it's not meant for expressions.

Does it seem that the process is actually taking place at this location?

polars/py-polars/polars/functions/eager.py

Lines 252 to 253 in 9ceedde

	elif isinstance(first, pl.Expr):
	return wrap_expr(plr.concat_expr([e._pyexpr for e in elems], rechunk))

[eitsupi]

That was added in #3541

[ritchie46]

Ah, my bad. Then it should. :)

[ritchie46]

Can the docstring be updated?