code: def test_where_columns_casting():
      # GH 42295
  
      df = DataFrame({"a": [1.0, 2.0], "b": [3, np.nan]})
      expected = df.copy()
      result = df.where(pd.notnull(df), None)
      # make sure dtypes don't change
      tm.assert_frame_equal(expected, result)
  
imports: from datetime import datetime
  from hypothesis import given
  import numpy
  import pytest
  from pandas.core.dtypes.common import is_scalar
  import pandas
  from pandas import DataFrame, DatetimeIndex, Index, Series, StringDtype, Timestamp, date_range, isna
  import pandas._testing
  from pandas._testing._hypothesis import OPTIONAL_ONE_OF_ALL
label_desc: Use image node_id 9 for calling a global function with example usage: test_where_columns_casting() without return types