code: def test_midx_losing_dtype():
      # GH#49830
      midx = MultiIndex.from_arrays([[0, 0], [np.nan, np.nan]])
      midx2 = MultiIndex.from_arrays([[1, 1], [np.nan, np.nan]])
      df1 = DataFrame({"a": [None, 4]}, index=midx)
      df2 = DataFrame({"a": [3, 3]}, index=midx2)
      result = df1.combine_first(df2)
      expected_midx = MultiIndex.from_arrays(
          [[0, 0, 1, 1], [np.nan, np.nan, np.nan, np.nan]]
      )
      expected = DataFrame(
          {"a": [np.nan, 4, 3, 3]}, index=expected_midx
      )
      tm.assert_frame_equal(result, expected)
  
imports: from datetime import datetime
  import numpy
  import pytest
  from pandas.core.dtypes.cast import find_common_type
  from pandas.core.dtypes.common import is_dtype_equal
  import pandas
  from pandas import DataFrame, Index, MultiIndex, Series
  import pandas._testing
label_desc: Use image node_id 7 for calling a global function with example usage: test_midx_losing_dtype() without return types