66 lines
1.3 KiB
Python
66 lines
1.3 KiB
Python
#!/usr/bin/env python
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# -*- encoding: utf-8 -*-
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"""
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Topic: 通过元类控制实例的创建
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Desc :
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"""
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class NoInstances(type):
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def __call__(self, *args, **kwargs):
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raise TypeError("Can't instantiate directly")
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# Example
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class Spam(metaclass=NoInstances):
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@staticmethod
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def grok(x):
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print('Spam.grok')
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class Singleton(type):
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def __init__(self, *args, **kwargs):
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self.__instance = None
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super().__init__(*args, **kwargs)
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def __call__(self, *args, **kwargs):
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if self.__instance is None:
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self.__instance = super().__call__(*args, **kwargs)
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return self.__instance
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else:
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return self.__instance
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# Example
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class Spam1(metaclass=Singleton):
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def __init__(self):
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print('Creating Spam')
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import weakref
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class Cached(type):
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def __init__(self, *args, **kwargs):
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super().__init__(*args, **kwargs)
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self.__cache = weakref.WeakValueDictionary()
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def __call__(self, *args):
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if args in self.__cache:
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return self.__cache[args]
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else:
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obj = super().__call__(*args)
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self.__cache[args] = obj
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return obj
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# Example
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class Spam2(metaclass=Cached):
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def __init__(self, name):
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print('Creating Spam({!r})'.format(name))
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self.name = name
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