code: def testfind_path_start(self):
      find_path_start = nx.algorithms.euler._find_path_start
      # Test digraphs return correct starting node.
      G = nx.path_graph(6, create_using=nx.DiGraph)
      assert find_path_start(G) == 0
      edges = [(0, 1), (1, 2), (2, 0), (4, 0)]
      assert find_path_start(nx.DiGraph(edges)) == 4
  
      # Test graph with no Eulerian path return None.
      edges = [(0, 1), (1, 2), (2, 3), (2, 4)]
      assert find_path_start(nx.DiGraph(edges)) is None
  
imports: import collections
  import pytest
  import networkx
label_desc: Use image node_id 1 for calling the TestFindPathStart obj's underlying member method code with example usage: obj.testfind_path_start() without return types