__eq__如何在Python中以什么顺序处理? [英] How is __eq__ handled in Python and in what order?

问题描述

由于Python不提供其比较运算符的左/右版本，因此它如何决定调用哪个函数?

Since Python does not provide left/right versions of its comparison operators, how does it decide which function to call?

class A(object):
    def __eq__(self, other):
        print "A __eq__ called"
        return self.value == other
class B(object):
    def __eq__(self, other):
        print "B __eq__ called"
        return self.value == other

>>> a = A()
>>> a.value = 3
>>> b = B()
>>> b.value = 4
>>> a == b
"A __eq__ called"
"B __eq__ called"
False

这似乎同时调用了两个__eq__函数.只是在寻找官方决策树.

This seems to call both __eq__ functions. Just looking for the official decision tree.

推荐答案

a == b表达式调用A.__eq__，因为它存在.其代码包括self.value == other.由于int不知道如何将自己与B进行比较，因此Python尝试调用B.__eq__来查看其是否知道如何将自己与int进行比较.

The a == b expression invokes A.__eq__, since it exists. Its code includes self.value == other. Since int's don't know how to compare themselves to B's, Python tries invoking B.__eq__ to see if it knows how to compare itself to an int.

如果您修改代码以显示正在比较的值:

If you amend your code to show what values are being compared:

class A(object):
    def __eq__(self, other):
        print("A __eq__ called: %r == %r ?" % (self, other))
        return self.value == other
class B(object):
    def __eq__(self, other):
        print("B __eq__ called: %r == %r ?" % (self, other))
        return self.value == other

a = A()
a.value = 3
b = B()
b.value = 4
a == b

它将打印:

A __eq__ called: <__main__.A object at 0x013BA070> == <__main__.B object at 0x013BA090> ?
B __eq__ called: <__main__.B object at 0x013BA090> == 3 ?

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