__eq__ 在 Python 中是如何处理的以及以什么顺序处理？

How is __eq__ handled in Python and in what order?

提问by PyProg

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

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

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

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

这似乎调用了这两个__eq__函数。只是在寻找官方的决策树。

采纳答案by Ned Batchelder

The a == bexpression 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.

的a == b表达调用A.__eq__，因为它的存在。它的代码包括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__以查看它是否知道如何将自己与 int 进行比较。

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

如果您修改代码以显示正在比较的值：

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

it will print:

它会打印：

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

回答by kev

When Python2.x sees a == b, it tries the following.

当 Python2.x 看到 时a == b，它会尝试以下操作。

• If type(b)is a new-style class, and type(b)is a subclass of type(a), and type(b)has overridden __eq__, then the result is b.__eq__(a).
• If type(a)has overridden __eq__(that is, type(a).__eq__isn't object.__eq__), then the result is a.__eq__(b).
• If type(b)has overridden __eq__, then the result is b.__eq__(a).
• If none of the above are the case, Python repeats the process looking for __cmp__. If it exists, the objects are equal iff it returns zero.
• As a final fallback, Python calls object.__eq__(a, b), which is Trueiff aand bare the same object.

• 如果type(b)是新式类，并且type(b)是 的子类type(a)，并且type(b)已覆盖__eq__，则结果为b.__eq__(a)。
• 如果type(a)已覆盖__eq__（即type(a).__eq__不是object.__eq__），则结果为a.__eq__(b)。
• 如果type(b)已覆盖__eq__，则结果为b.__eq__(a)。
• 如果上述情况都不是，Python 会重复查找__cmp__. 如果它存在，则对象相等且仅当它返回 时zero。
• 作为最后的回退，Python 调用object.__eq__(a, b)，它是Trueiffa并且b是同一个对象。

If any of the special methods return NotImplemented, Python acts as though the method didn't exist.

如果任何特殊方法 return NotImplemented，Python 就好像该方法不存在一样。

Note that last step carefully: if neither anor boverloads ==, then a == bis the same as a is b.

请注意最后一步：如果既不是a也不是b重载==，则a == b与 相同a is b。

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From https://eev.ee/blog/2012/03/24/python-faq-equality/

来自https://eev.ee/blog/2012/03/24/python-faq-equality/