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.

def get_vertex(self, key):
    """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.
    """
    if key in self.vertex_map:
      return self.vertex_map[key]
    vertex = self.new_vertex()
    self.vertex_map[key] = vertex
    return vertex