关于比较：如何在Python中处理__eq__以及以什么顺序处理？

How is eq handled in Python and in what order?

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？

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

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

这似乎调用了两个__eq__函数。只是在寻找官方的决策树。

a == b表达式调用A.__eq__，因为它存在。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，所以python尝试调用B.__eq__来查看它是否知道如何将自己与int进行比较。

如果修改代码以显示正在比较的值：

1
2
3
4
5
6
7
8
9
10
11
12
13
14

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：

1 2	A __eq__ called: <__main__.A object at 0x013BA070> == <__main__.B object at 0x013BA090> ? B __eq__ called: <__main__.B object at 0x013BA090> == 3 ?

码农家园

关于比较：如何在Python中处理eq以及以什么顺序处理？

How is eq handled in Python and in what order?

How is __eq__ handled in Python and in what order?

How is eq handled in Python and in what order?