41 lines
944 B
Python
41 lines
944 B
Python
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"""
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混入 - Mix-in
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限制字典只有在指定的key不存在时才能设置键值对
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原则上能够不使用多重继承的地方都不要用多重继承
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MRO - Method Resolution Order - 方法解析顺序
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Python2 - 深度优先搜索
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Python3 - 类似于广度优先搜索 - C3算法
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类.__mro__ / 类.mro()
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"""
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class SetOnceMappingMixin():
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"""混入类"""
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__slots__ = ()
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def __setitem__(self, key, value):
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if key in self:
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raise KeyError(f'键{str(key)}已经存在')
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return super().__setitem__(key, value)
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class SetOnceDict(SetOnceMappingMixin, dict):
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"""自定义字典"""
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pass
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def main():
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"""主函数"""
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dict1 = SetOnceDict()
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try:
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dict1['username'] = 'jackfrued'
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dict1['username'] = 'hellokitty'
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dict1['username'] = 'wangdachui'
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except KeyError as error:
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print('Error:', error)
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print(dict1)
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if __name__ == '__main__':
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main()
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