python - 使用 __hash__ 函数设置包含用户定义的类

Question

给定:

class T:
    def __hash__(self):
        return 1234

t1 = T()
t2 = T()

my_set = { t1 }

我希望打印以下内容True:

print t2 in my_set

这不是应该打印 True 因为 t1 和 t2 具有相同的哈希值。如何使 set 的 in 运算符使用给定的哈希函数？

Answer 1

您需要定义一个 __eq__ 方法，因为只有相同 a is b 或等于 a == b 的实例(除了具有相同的hash)将被set和dict识别为相等:

class T:
    def __hash__(self):
        return 1234
    def __eq__(self, other):
        return True

t1 = T()
t2 = T()

my_set = { t1 }

print(t2 in my_set)  # True

data model on __hash__ (和 the same documentation page for Python 2 )解释了这一点:

__hash__

Called by built-in function hash() and for operations on members of hashed collections including set, frozenset, and dict. __hash__() should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple.

If a class does not define an __eq__() method it should not define a __hash__() operation either; if it defines __eq__() but not __hash__(), its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an __eq__() method, it should not implement __hash__(), since the implementation of hashable collections requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket).

User-defined classes have __eq__() and __hash__() methods by default; with them, all objects compare unequal (except with themselves) and x.__hash__() returns an appropriate value such that x == y implies both that x is y and hash(x) == hash(y).

(强调我的)

注意:在 Python 2 中，您还可以实现 __cmp__ 方法而不是 __eq__。