如果使用Python进行鸭式打字，您是否应该测试isinstance? [英] If duck-typing in Python, should you test isinstance?

问题描述

您有一个需要相等测试的Python类. Python应该使用鸭式输入法，但是在 eq 函数中包含或排除isinstance测试(更好/更准确)是吗?例如:

You have a Python class which needs an equals test. Python should use duck-typing but is it (better/more accurate) to include or exclude an isinstance test in the eq function? For example:

class Trout(object):
    def __init__(self, value):
        self.value = value

    def __eq__(self, other):
        return isinstance(other, Trout) and self.value == other.value

推荐答案

在__eq__方法中使用isinstance很常见.这样做的原因是，如果__eq__方法失败，则它可能会从另一个对象回退到__eq__方法.大多数常规方法都被显式调用，但__eq__被隐式调用，因此它需要更频繁地跳转.

Using isinstance in __eq__ methods is pretty common. The reason for this is that if the __eq__ method fails, it can fallback on an __eq__ method from another object. Most normal methods are called explicitly, but __eq__ is called implicitly, so it requires look-before-you-leap more frequently.

编辑(感谢提醒，斯文·马纳赫):

EDIT (thanks for the reminder, Sven Marnach):

要使其回退，可以返回NotImplemented单例，如本例所示:

To make it fallback, you can return the NotImplemented singleton, as in this example:

class Trout(object):
    def __init__(self, value):
        self.value = value

    def __eq__(self, other):
        if isinstance(other, Trout):
            return self.value == other.value
        else:
            return NotImplemented

假设RainbowTrout知道如何将自己与Trout或另一个RainbowTrout进行比较，但是Trout仅知道如何将自身与Trout进行比较.在此示例中，如果测试mytrout == myrainbowtrout，Python将首先调用mytrout.__eq__(myrainbowtrout)，注意它失败，然后调用myrainbowtrout.__eq__(mytrout)，这将成功.

Suppose a RainbowTrout knows how to compare itself to a Trout or to another RainbowTrout, but a Trout only knows how to compare itself to a Trout. In this example, if you test mytrout == myrainbowtrout, Python will first call mytrout.__eq__(myrainbowtrout), notice that it fails, and then call myrainbowtrout.__eq__(mytrout), which succeeds.

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