code: def test_merge_na_keys(self):
      data = [
          [1950, "A", 1.5],
          [1950, "B", 1.5],
          [1955, "B", 1.5],
          [1960, "B", np.nan],
          [1970, "B", 4.0],
          [1950, "C", 4.0],
          [1960, "C", np.nan],
          [1965, "C", 3.0],
          [1970, "C", 4.0],
      ]
  
      frame = DataFrame(data, columns=["year", "panel", "data"])
  
      other_data = [
          [1960, "A", np.nan],
          [1970, "A", np.nan],
          [1955, "A", np.nan],
          [1965, "A", np.nan],
          [1965, "B", np.nan],
          [1955, "C", np.nan],
      ]
      other = DataFrame(other_data, columns=["year", "panel", "data"])
  
      result = frame.merge(other, how="outer")
  
      expected = frame.fillna(-999).merge(
          other.fillna(-999), how="outer"
      )
      expected = expected.replace(-999, np.nan)
  
      tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  import pandas.util._test_decorators
  import pandas
  from pandas import DataFrame, Index, MultiIndex, RangeIndex, Series, Timestamp, option_context
  import pandas._testing
  from pandas.core.reshape.concat import concat
  from pandas.core.reshape.merge import merge
label_desc: Use image node_id 10 for calling the TestMergeMulti obj's underlying member method code with example usage: obj.test_merge_na_keys() without return types