code: def get_vertex(self, key):
      
      if key in self.vertex_map:
        return self.vertex_map[key]
      vertex = self.new_vertex()
      self.vertex_map[key] = vertex
      return vertex
docs: Returns or Creates a Vertex mapped by key.
  
      Args:
        key: A string reference for a vertex.  May refer to a new Vertex in which
        case it will be created.
  
      Returns:
        A the Vertex mapped to by key.