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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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