code: def test_degree_mixing_dict_directed(self):
      d = nx.degree_mixing_dict(self.D)
      print(d)
      d_result = {1: {3: 2}, 2: {1: 1, 3: 1}, 3: {}}
      assert d == d_result
  
imports: import pytest
  import networkx
  from .base_test import BaseTestAttributeMixing, BaseTestDegreeMixing
label_desc: Use image node_id 3 for calling the TestDegreeMixingDict obj's underlying member method code with example usage: obj.test_degree_mixing_dict_directed() without return types