欺骗Python的运算符优先级

Question

我知道，当我比较两个物体lhs == rhs两者定义__eq__，只是lhs.__eq__被称为除非它返回NotImplemented或rhs是lhs一个子类。

但是我想实现一个类，在与任意对象进行比较时，它们的实例将有机会说出他们想说的内容，而不管arbitrary_object.__eq__的实现细节以及比较语句中的位置如何。听起来有些尴尬，但我正在开发一个面向测试的项目，看看testmania.expect，你就会明白我需要的是什么。

我最初的想法是让我的课是使用元类魔法和__instancecheck__，__subclasscheck__任何其他类的子类。但他们根本不会在简单比较的情况下被调用。

有人有什么新鲜的想法吗？

Answer 1

我不知道这是否适合您的需要，但为什么不测试这两个操作，我的意思是测试如果：object1 == object2和object2 == object1，通常你应该最终得到相同的值，除非其中一个对象被覆盖该__eq__方法，因此将执行这一新__eq__方法，并返回一个是真的，一个例子是胜于言：

def _assert_just_now(first, second): 
    """A Dump function to simulate if two dates are almost equal. 

    N.B: In this Dump function i will just test if the two datetime object have the 
    same hour 

    """ 

    from datetime import datetime 
    assert isinstance(first, datetime) and isinstance(second, datetime), \ 
      "This function only accept datetime objects" 

    return first.hour == second.hour 

class Expectation(object): 

    def __init__(self, assertion, first): 
     self.assertion = assertion 
     self.first = first 

    def __eq__(self, other): 
     return self.assertion(self.first, other) 

def assert_equal(first, second): 
    """Usage : 

    >>> from datetime import datetime 
    >>> t1 = datetime(year=2007, hour=1, month=3, day=12) 
    >>> t2 = datetime(year=2011, hour=1, month=5, day=12) 

    Without using Expectation it's False. 
    >>> assert_equal(t1, t2) 
    False 

    Use the Expectation object. 
    >>> assert_equal(t1, Expectation(_assert_just_now, t2)) 
    True 

    Can use Expectation in the first argument too. 
    >>> assert_equal(Expectation(_assert_just_now, t2), t1) 
    True 

    Work also in Container object. 
    >>> assert_equal({'a': 1, 'b': Expectation(_assert_just_now, t2)}, 
    ...    {'a': 1, 'b': t1}) 
    True 

    We change a little bit the values to make the assert equal fail. 
    >>> t3 = datetime(year=2011, hour=2, month=5, day=12) 
    >>> assert_equal(t1, t3) 
    False 

    This just to make sure that the _assert_just_now doesn't accept object 
    other than datetime: 
    >>> assert_equal(t1, Expectation(_assert_just_now, "str")) 
    Traceback (most recent call last): 
     ... 
    AssertionError: This function only accept datetime objects 

    """ 

    return first == second or second == first 

if __name__ == '__main__': 
    import doctest 
    doctest.testmod() 

希望这可以帮助。