code: def test_combine_preserve_dtypes():
      # GH7509
      a_column = Series(["a", "b"], index=range(2))
      b_column = Series(range(2), index=range(2))
      df1 = DataFrame({"A": a_column, "B": b_column})
  
      c_column = Series(["a", "b"], index=range(5, 7))
      b_column = Series(range(-1, 1), index=range(5, 7))
      df2 = DataFrame({"B": b_column, "C": c_column})
  
      expected = DataFrame(
          {
              "A": ["a", "b", np.nan, np.nan],
              "B": [0, 1, -1, 0],
              "C": [np.nan, np.nan, "a", "b"],
          },
          index=[0, 1, 5, 6],
      )
      combined = df1.combine_first(df2)
      tm.assert_frame_equal(combined, 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 4 for calling a global function with example usage: test_combine_preserve_dtypes() without return types