code: def test_negative_edge_cycle_custom_weight_key(self):
      d = nx.DiGraph()
      d.add_edge("a", "b", w=-2)
      d.add_edge("b", "a", w=-1)
      assert nx.negative_edge_cycle(d, weight="w")
  
imports: import pytest
  import networkx
  from networkx.utils import pairwise
label_desc: Use image node_id 14 for calling the TestWeightedPath obj's underlying member method code with example usage: obj.test_negative_edge_cycle_custom_weight_key() without return types