[Bug]: Filter in agg context

[IsaiasGutierrezCruz]

Describe the bug

When the filter expression is used within an agg context using a pandas dataframe, the following exception is raised:

ValueError: Length of values (1) does not match length of index (0)

I was also evaluating the use of a when-otherwise expression to emulate the behavior of filtering, but anonymous expressions are not supported in the agg context. Is there a plan to support this kind of expression in that context? c:

Steps or code to reproduce the bug

def replicate_error_filter() -> None:
    data = {
        "cat": ["a", "a", "a", "b", "b", "b", "c", "c", "c"],
        "cat_2": ["d", "d", "e", "e", "f", "f", "g", "g", "g"],
        'values': [1, 2, 3, 4, 5, 6, 7, 8, 10]
    }
    original_data = pd.DataFrame(data)
    df_nw = nw.from_native(original_data)
    df_nw = (
        df_nw
        .group_by(
            'cat'
        )
        .agg(
            nw.col("values")
            .filter(nw.col("cat_2") == "d")
            .max()
        )
    )
    print(df_nw.to_native())

Expected results

┌─────┬────────┐
│ cat ┆ values │
│ --- ┆ ---    │
│ str ┆ i64    │
╞═════╪════════╡
│ a   ┆ 2      │
│ b   ┆ null   │
│ c   ┆ null   │
└─────┴────────┘

Actual results

ValueError: Length of values (1) does not match length of index (0)

Please run narwhals.show_version() and enter the output below.

System:
    python: 3.12.5 (main, Aug 14 2024, 04:32:18) [Clang 18.1.8 ]
executable: /Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/bin/python
   machine: macOS-15.0.1-arm64-arm-64bit

Python dependencies:
     narwhals: 1.9.3
       pandas: 2.2.3
       polars: 1.8.2
         cudf: 
        modin: 
      pyarrow: 17.0.0
        numpy: 2.1.1

Relevant log output

File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/narwhals/_pandas_like/group_by.py", line 246, in func
    results_keys = expr._call(from_dataframe(df))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/narwhals/_expression_parsing.py", line 232, in func
    out.append(plx._create_series_from_scalar(_out, column))  # type: ignore[arg-type]
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/narwhals/_pandas_like/namespace.py", line 72, in _create_series_from_scalar
    return PandasLikeSeries._from_iterable(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/narwhals/_pandas_like/series.py", line 152, in _from_iterable
    native_series_from_iterable(
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/narwhals/_pandas_like/utils.py", line 185, in native_series_from_iterable
    return implementation.to_native_namespace().Series(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/pandas/core/series.py", line 575, in __init__
    com.require_length_match(data, index)
  File "/Users/abelisaiasgutierrezcruz/Documents/Proyects/test_narwhals/.venv/lib/python3.12/site-packages/pandas/core/common.py", line 573, in require_length_match
    raise ValueError(
ValueError: Length of values (1) does not match length of index (0)

[MarcoGorelli]

thanks @IsaiasGutierrezCruz for the report!

anonymous expressions are not supported in the agg context. Is there a plan to support this kind of expression in that context?

I think it would be tricky to do in the general case, at least for pandas/pyarrow/dask. but in #299 there is a suggestion to at least support more cases than we currently do