code: def test_period_in_column_name():
      df_period = pd.DataFrame([[1, 2]], columns=["normal", "a.b"])
      spec = period_in_col_name_spec()
      datasets, warnings = vf.runtime.pre_transform_datasets(
          spec, ["data_0"], inline_datasets=dict(df_period=df_period)
      )
      assert len(warnings) == 0
      assert len(datasets) == 1
  
      dataset = datasets[0]
      assert dataset.to_dict("records") == [{"normal": 1, "a.b": 2}]
  
imports: import pandas
  import pytest
  from pandas import Timestamp, NaT
  import pyarrow
  import vegafusion
  import json
  import polars
  from datetime import date
  import decimal
  import base64
label_desc: Use image node_id 20 for calling a global function with example usage: test_period_in_column_name() without return types