1、__eq__方法返回NotImplemented的情况

None.__eq__('a')

通过None直接调用__eq__方法返回的是NotImplemented，(1).__eq__('a')和'a'.__eq__(1)也返回NotImplemented。

2、示例代码 

class A:
    def __eq__(self, other):
        print('A.__eq__')
        return NotImplemented
class B:
    def __eq__(self, other):
        print('B.__eq__')
        return NotImplemented
class C:
    def __eq__(self, other):
        print('C.__eq__')
        return True
a = A()
b = B()
c = C()
print(a == b)
# A.__eq__
# B.__eq__
# False
print(a == c)
# A.__eq__
# C.__eq__
# True
print(c == a)
# C.__eq__
# True

3、结论

None.__eq__("a")等于True的情况，由于None.__eq__("a")返回NotImplemented
bool(NotImplemented)是True,所以bool(None.__eq__("a"))就等于True。这种bool(NotImplemented)的特殊情况，值得注意一下。