code: def test_degree_mixing_dict_undirected(self):
      d = nx.degree_mixing_dict(self.P4)
      d_result = {1: {2: 2}, 2: {1: 2, 2: 2}}
      assert d == d_result
  
imports: import pytest
  import networkx
  from .base_test import BaseTestAttributeMixing, BaseTestDegreeMixing
label_desc: Use image node_id 1 for calling the TestDegreeMixingDict obj's underlying member method code with example usage: obj.test_degree_mixing_dict_undirected() without return types