Python中的IdentitySet？

from collections import MutableSet

class IdentitySet(MutableSet):
    key = id  # should return a hashable object

    def __init__(self,iterable=()):
        self.map = {} # id -> object
        self |= iterable  # add elements from iterable to the set (union)

    def __len__(self):  # Sized
        return len(self.map)

    def __iter__(self):  # Iterable
        return self.map.itervalues()

    def __contains__(self,x):  # Container
        return self.key(x) in self.map

    def add(self,value):  # MutableSet
        """Add an element."""
        self.map[self.key(value)] = value

    def discard(self,value):  # MutableSet
        """Remove an element.  Do not raise an exception if absent."""
        self.map.pop(self.key(value),None)

    def __repr__(self):
        if not self:
            return '%s()' % (self.__class__.__name__,)
        return '%s(%r)' % (self.__class__.__name__,list(self))

例：

a = (1,2)
print IdentitySet([a,(1,2),a])
# -> IdentitySet([(1,2)]) # only one instance of `a`

print s != Set([a,a])  # it might be unequal because new literal
                                # tuple (1,2) might have a different id
print s | Set([a,2)]) # `a` plus two tuples from literals

MutableSet自动提供了一些方法：清除,流行,删除,__ior__,__iand__,__ixor__,__isub__,__le__,__lt__,__eq__,__ne__,__gt__,__ge__,__and__,__or__,__sub__,__xor__和isdisjoint.

这里,与基于集合的实现不同,复合操作总是使用重写的基本方法,| (self .__或__(),union)是根据self.add()实现的,因此它为IdentitySet语义提供了正确的.