如何使用__getattr__委托方法进行属性分配? [英] How to use __getattr__ to delegate methods to attribute?

问题描述

我有以下课程:

class MyInt:
    def __init__(self, v):
        if type(v) != int:
            raise ValueError('value must be an int')
        self.v = v

    def __getattr__(self, attr):
        return getattr(self.v, attr)

i = MyInt(0)
print(i + 1)

我收到错误:TypeError: unsupported operand type(s) for +: 'MyInt' and 'int'

不应该叫i.__add__(1)吗?当在MyInt类中找不到这样的方法时，不应该调用__getattr__吗?

Shouldn't i.__add__(1) be called? And shouldn't __getattr__ be called when no such method is found in the MyInt class?

推荐答案

__getattr__不能用于生成其他魔术方法.您需要单独实现所有这些功能.

__getattr__ cannot be used to generate other magic methods. You'll need to implement all of them individually.

当Python语言内部人员查询诸如__add__之类的魔术方法时，它们会完全绕过__getattr__，__getattribute__和实例字典.查找大致像

When the Python language internals look up magic methods like __add__, they completely bypass __getattr__, __getattribute__, and the instance dict. The lookup goes roughly like

def find_magic_method(object, method_name):
    for klass in type(object).__mro__:
        if method_name in klass.__dict__:
            return klass.__dict__[method_name]
    raise AttributeError

如果要查看确切的查找过程，请在

If you want to see the exact lookup procedure, it's _PyObject_LookupSpecial in Objects/typeobject.c.

如果您想知道Python为什么要这样做，那么有很多魔术方法很难做到或根本无法完成您所期望的事情.例如，Python可能无法使用__getattribute__查找__getattribute__，因为这将导致无基数的无限递归.

If you're wondering why Python does this, there are a lot of magic methods for which it would be really awkward or impossible to do what you were expecting. For example, Python couldn't possibly use __getattribute__ to look up __getattribute__, as that would cause infinite recursion with no base case.

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