在Python中，我可以使用对象（类的实例）作为字典键吗？

Question

我想使用类实例作为字典键，例如：

classinstance = class()
dictionary[classinstance] = 'hello world'

Python似乎无法将类作为字典键处理，或者我错了？ 此外，我可以使用类似于[（classinstance，helloworld），…]的元组列表来代替字典，但这看起来非常不专业。 你有解决那个问题的线索吗？

Answer 1

以下代码运行良好，因为默认情况下，类对象是散列的：

Class Foo(object):
    def __init__(self):
        pass

myinstance = Foo()
mydict = {myinstance : 'Hello world'}

print mydict[myinstance]

输出： 你好，世界

此外，对于更高级的用法，您应该阅读以下文章：

Object of custom type as dictionary key

Answer 2

你的实例需要是散列的。python glossary告诉我们：

An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() or __cmp__() method). Hashable objects which compare equal must have the same hash value.

Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

All of Python’s immutable built-in objects are hashable, while no mutable containers (such as lists or dictionaries) are. Objects which are instances of user-defined classes are hashable by default; they all compare unequal, and their hash value is their id().

Answer 3

尝试在类中实现hash和eq方法。

例如，下面是我创建的一个简单的哈希字典类：

class hashable_dict:
    def __init__(self, d):
        self.my_dict = d
        self.my_frozenset = frozenset(d.items())
    def __getitem__(self, item):
        return self.my_dict[item]
    def __hash__(self):
        return hash(self.my_frozenset)
    def __eq__(self, rhs):
        return isinstance(rhs, hashable_dict) and self.my_frozenset == rhs.my_frozenset
    def __ne__(self, rhs):
       return not self == rhs
    def __str__(self):
        return 'hashable_dict(' + str(self.my_dict) + ')'
    def __repr__(self):
        return self.__str__()