code: def test_margin_dropna3(self):
      df = DataFrame(
          {
              "a": [1, np.nan, np.nan, np.nan, np.nan, 2],
              "b": [3, 3, 4, 4, 4, 4],
          }
      )
      actual = crosstab(df.a, df.b, margins=True, dropna=True)
      expected = DataFrame([[1, 0, 1], [0, 1, 1], [1, 1, 2]])
      expected.index = Index([1.0, 2.0, "All"], name="a")
      expected.columns = Index([3, 4, "All"], name="b")
      tm.assert_frame_equal(actual, expected)
  
imports: import numpy
  import pytest
  import pandas
  from pandas import CategoricalDtype, CategoricalIndex, DataFrame, Index, MultiIndex, Series, crosstab
  import pandas._testing
label_desc: Use image node_id 12 for calling the TestCrosstab obj's underlying member method code with example usage: obj.test_margin_dropna3() without return types