code: def testHash(self):
      data = {}
      n = self.g.vcount()
      for i in range(n):
          code1 = hash(self.g.vs[i])
          code2 = hash(self.g.vs[i])
          self.assertEqual(code1, code2)
          data[self.g.vs[i]] = i
  
      for i in range(n):
          self.assertEqual(i, data[self.g.vs[i]])
  
imports: import unittest
  from igraph import Graph, Vertex, VertexSeq
  from .utils import is_pypy
label_desc: Use image node_id 2 for calling the VertexTests obj's underlying member method code with example usage: obj.testHash() without return types