code: def _find_path_start(G):
      """Return a suitable starting vertex for an Eulerian path.
  
      If no path exists, return None.
      """
      if not has_eulerian_path(G):
          return None
  
      if is_eulerian(G):
          return arbitrary_element(G)
  
      if G.is_directed():
          v1, v2 = (v for v in G if G.in_degree(v) != G.out_degree(v))
          # Determines which is the 'start' node (as opposed to the 'end')
          if G.out_degree(v1) > G.in_degree(v1):
              return v1
          else:
              return v2
  
      else:
          # In an undirected graph randomly choose one of the possibilities
          start = [v for v in G if G.degree(v) % 2 != 0][0]
          return start
  
file_path: networkx/networkx/algorithms/euler.py
imports: from itertools import combinations
  import networkx
  from ..utils import arbitrary_element, not_implemented_for
label_desc: Use image node_id 3 for calling a global function with example usage: _find_path_start(G) and returns: None, arbitrary_element, start, v1, v2