Question

我想创建一个数据结构, 它像字典一样, 具有一个附加功能, 以记录哪些密钥已被“ 消耗 ” 。请注意, 我无法在重新使用时弹出这些值。

该结构应支持这三种情况,即当进入时将键的用量标为:

if key in d:
    ...
d[key]
d.get(key)

这是我所写的:

class DictWithMemory(dict):

    def __init__(self, *args, **kwargs):
        self.memory = set()
        return super(DictWithMemory, self).__init__(*args, **kwargs)

    def __getitem__(self, key):
        self.memory.add(key)
        return super(DictWithMemory, self).__getitem__(key)

    def __contains__(self, key):
        self.memory.add(key)
        return super(DictWithMemory, self).__contains__(key)

    def get(self, key, d=None):
        self.memory.add(key)
        return super(DictWithMemory, self).get(key, d)

    def unused_keys(self):
        """
        Returns the list of unused keys.
        """
        return set(self.keys()).difference(self.memory)

由于我不太熟悉法令的内涵,是否有更好的办法取得这一结果?

Answer 1

这里的解决方案可以将所有东西在元类中摘要化。我不确定这是否真的更优雅, 但如果您改变想法, 如何储存旧密钥, 它确实能提供一些封装 :

class KeyRememberer(type):

    def __new__(meta, classname, bases, classDict):
        cls = type.__new__(meta, classname, bases, classDict)

        # Define init that creates the set of remembered keys
        def __init__(self, *args, **kwargs):
            self.memory = set()
            return super(cls, self).__init__(*args, **kwargs)
        cls.__init__ = __init__

        # Decorator that stores a requested key in the cache
        def remember(f):
            def _(self, key, *args, **kwargs):
                self.memory.add(key)
                return f(self, key, *args, **kwargs)
            return _

        # Apply the decorator to each of the default implementations
        for method_name in [   __getitem__ ,  __contains__ ,  get  ]:
            m = getattr(cls, method_name)
            setattr(cls, method_name, remember(m))

        return cls


class DictWithMemory(dict):

    # A metaclass that ensures the object
    # has a set called  memory  as an attribute,
    # which is updated on each call to __getitem__,
    # __contains__, or get.
    __metaclass__ = KeyRememberer

    def unused_keys(self):
        """
        Returns the list of unused keys.
        """
        print "Used", self.memory
        return list(set(super(DictWithMemory,
                              self).keys()).difference(self.memory))

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