code: def test_group_by_all_masked_out() -> None:
      df = pl.DataFrame(
          {
              "val": pl.Series(
                  [None, None, None, None],
                  dtype=pl.Categorical,
                  nan_to_null=True,
              ).set_sorted(),
              "col": [4, 4, 4, 4],
          }
      )
      parts = df.partition_by("val")
      assert len(parts) == 1
      assert_frame_equal(parts[0], df)
  
imports: from __future__ import annotations
  from collections import OrderedDict
  from datetime import datetime, timedelta
  from typing import Any
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 20 for calling a global function with example usage: test_group_by_all_masked_out() without return types