Raise informative error message when group_by(by=[...]) is used

[barak1412]

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({
    'a': [0, 1, 1, 2, 2],
    'b': [0, 0, 1, 2, 2],
    'v': [9, 5, 4, 8, 3]
})

df_grouped = df.group_by(by=['a', 'b']).agg(pl.sum('v'))
print(df_grouped)

Results in:

shape: (1, 2)
┌────────────┬─────┐
│ by         ┆ v   │
│ ---        ┆ --- │
│ list[str]  ┆ i64 │
╞════════════╪═════╡
│ ["a", "b"] ┆ 29  │
└────────────┴─────┘

However, the following:

import polars as pl


df = pl.DataFrame({
    'a': [0, 1, 1, 2, 2],
    'b': [0, 0, 1, 2, 2],
    'v': [9, 5, 4, 8, 3]
})

df_grouped = df.group_by(by=['a', 'b']).agg(pl.sum('v'))
print(df_grouped)

Results in:

shape: (4, 3)
┌─────┬─────┬─────┐
│ a   ┆ b   ┆ v   │
│ --- ┆ --- ┆ --- │
│ i64 ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ 2   ┆ 2   ┆ 11  │
│ 1   ┆ 0   ┆ 5   │
│ 0   ┆ 0   ┆ 9   │
│ 1   ┆ 1   ┆ 4   │
└─────┴─────┴─────┘

Log output

No response

Issue description

When using the by kwarg, we get List type as column.

Expected behavior

I would expect using the by kwarg to yeild the same result.

Installed versions

--------Version info---------
Polars:               1.1.0
Index type:           UInt32
Platform:             Windows-10-10.0.19041-SP0
Python:               3.8.1 (tags/v3.8.1:1b293b6, Dec 18 2019, 23:11:46) [MSC v.1916 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          2.2.0
connectorx:           0.3.3a1
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2023.10.0
gevent:               <not installed>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           3.5.1
nest_asyncio:         1.5.1
numpy:                1.24.4
openpyxl:             <not installed>
pandas:               2.0.3
pyarrow:              13.0.0
pydantic:             1.8.2
pyiceberg:            <not installed>
sqlalchemy:           1.4.32
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[deanm0000]

repeat of this.

[barak1412]

@deanm0000 I saw @ritchie46 comment there about using kwarg as syntactic suger, but it still feels very odd to get a list type when 2 columns are supplied, as result.

Are you sure it is intended?

[MarcoGorelli]

yeah, the list just gets interpreted as a literal, just like how it would here:

In [15]: df = pl.DataFrame({'a': [1,2,3], 'b': [4,5,6]})

In [16]: df.with_columns(by=['a', 'b'])
Out[16]:
shape: (3, 3)
┌─────┬─────┬────────────┐
│ a   ┆ b   ┆ by         │
│ --- ┆ --- ┆ ---        │
│ i64 ┆ i64 ┆ list[str]  │
╞═════╪═════╪════════════╡
│ 1   ┆ 4   ┆ ["a", "b"] │
│ 2   ┆ 5   ┆ ["a", "b"] │
│ 3   ┆ 6   ┆ ["a", "b"] │
└─────┴─────┴────────────┘

[MarcoGorelli]

having said that, there's no valid use case for grouping by a list literal...I think it might be worthwhile to raise an informative error here

[barak1412]

@MarcoGorelli Agree

[deanm0000]

It is intended that you don't use by as a named argument rather than positional. Like I said, I don't know what is causing the difference in behavior so I don't think it is explicitly intended that the behavior changed just that the old behavior was more of an accident for lack of a better term.