如何在图形中实现add_vertex和add_edge函数？

class Vertex:
    def __init__(self, value):
        self.value = value

class Edge:
    def __init__(self, vertex1, vertex2):
        self.vertex1 = vertex1
        self.vertex2 = vertex2

class Graph:
    def __init__(self):
        self.vertices = []
        self.edges = []

    def add_vertex(self, value):
        vertex = Vertex(value)
        self.vertices.append(vertex)

    def add_edge(self, value1, value2):
        vertex1 = None
        vertex2 = None
        for vertex in self.vertices:
            if vertex.value == value1:
                vertex1 = vertex
            if vertex.value == value2:
                vertex2 = vertex
        if vertex1 is not None and vertex2 is not None:
            edge = Edge(vertex1, vertex2)
            self.edges.append(edge)

# 创建一个图对象
graph = Graph()

# 添加顶点
graph.add_vertex("A")
graph.add_vertex("B")
graph.add_vertex("C")

# 添加边
graph.add_edge("A", "B")
graph.add_edge("B", "C")