为什么方法没有引用相等? [英] Why don't methods have reference equality?

问题描述

我遇到一个错误，即使用is时我依赖于彼此相等的方法.事实并非如此:

>>> class What:
...     def meth(self):
...         pass

>>> What.meth is What.meth
True
>>> inst = What()
>>> inst.meth is inst.meth
False

为什么会这样?它适用于常规功能:

>>> def func(): pass
>>> func is func
True

解决方案

每次访问它们时都会创建方法对象.函数充当描述符，当它们的.__get__方法称为:

>>> What.__dict__['meth']
<function What.meth at 0x10a6f9c80>
>>> What.__dict__['meth'].__get__(What(), What)
<bound method What.meth of <__main__.What object at 0x10a6f7b10>>

如果您使用的是Python 3.8或更高版本，则可以改用==相等性测试.在Python 3.8和更高版本上，如果两个方法的.__self__和.__func__属性是相同的对象(因此，如果它们包装相同的函数并绑定到相同的实例，并且都用is测试)，则这两个方法是相等的.

在3.8之前，根据方法的实现方式，方法==的行为是不一致的-Python方法和两种C方法类型之一比较__self__是否相等，而不是标识，而另一种C方法类型比较按身份.参见 Python问题1617161 .

如果需要测试这些方法表示相同的基础函数，请测试其__func__属性:

>>> What.meth == What.meth     # functions (or unbound methods in Python 2)
True
>>> What().meth == What.meth   # bound method and function
False
>>> What().meth == What().meth # bound methods with *different* instances
False
>>> What().meth.__func__ == What().meth.__func__ # functions
True

I had a bug where I was relying on methods being equal to each other when using is. It turns out that's not the case:

>>> class What:
...     def meth(self):
...         pass

>>> What.meth is What.meth
True
>>> inst = What()
>>> inst.meth is inst.meth
False

Why is that the case? It works for regular functions:

>>> def func(): pass
>>> func is func
True

解决方案

Method objects are created each time you access them. Functions act as descriptors, returning a method object when their .__get__ method is called:

>>> What.__dict__['meth']
<function What.meth at 0x10a6f9c80>
>>> What.__dict__['meth'].__get__(What(), What)
<bound method What.meth of <__main__.What object at 0x10a6f7b10>>

If you're on Python 3.8 or later, you can use == equality testing instead. On Python 3.8 and later, two methods are equal if their .__self__ and .__func__ attributes are identical objects (so if they wrap the same function, and are bound to the same instance, both tested with is).

Before 3.8, method == behaviour is inconsistent based on how the method was implemented - Python methods and one of the two C method types compare __self__ for equality instead of identity, while the other C method type compares __self__ by identity. See Python issue 1617161.

If you need to test that the methods represent the same underlying function, test their __func__ attributes:

>>> What.meth == What.meth     # functions (or unbound methods in Python 2)
True
>>> What().meth == What.meth   # bound method and function
False
>>> What().meth == What().meth # bound methods with *different* instances
False
>>> What().meth.__func__ == What().meth.__func__ # functions
True

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