python - 对于私有(private)属性，setattr 和赋值并不等效

Question

使用CPython 3.4.3(GCC 5.3.1 20160406(Red Hat 5.3.1-6))，我在使用setattr和getattr时遇到非常奇怪的行为关于私有(private)属性:

class Short:
    def __init__(self):
        setattr(self, '__myatt', None)
        self.__myatt = 42
        assert getattr(self, '__myatt') is None
Short()

最后的代码永远不会引发任何 AssertionError，尽管在​​断言语句之前有一些矫揉造作。但是，使用 myattr 而不是 __myatt 的相同代码可以正确引发异常:

class Short:
    def __init__(self):
        setattr(self, 'myatt', None)
        self.myatt = 42
        assert getattr(self, 'myatt') is 42
Short()

<小时/>

以下是两种情况(私有(private)与公共(public))的比较，带有注释和断言，显示 3 种访问方式中的 2 种没有返回预期值:

class Private:

    def __init__(self):
        print('############### PRIVATE ###############')
        print('Assign __fa with None using setattr.')
        setattr(self, '__fa', None)


        print("Set __fa to 42 using regular affectation.")
        new_value = 42
        self.__fa = new_value

        print("\nPrint the new values. Expected to be all 42:")
        print("new value:", new_value)
        print("self.__fa:", self.__fa)
        print("getattr(self, '__fa'):", getattr(self, '__fa'))
        print("self.__dict__['__fa']:", self.__dict__['__fa'])

        assert self.__dict__['__fa'] is None  # this is unexpected
        assert getattr(self, '__fa') is None  # this is unexpected

        print("\nNow modify __fa using setattr")
        # Maintenant, on utilise la notation «équivalente» (d'après la doc)
        setattr(self, '__fa', new_value)

        print("\nPrint the new values. Expected to be all 42:")
        # et tout va bien !
        # WTF !
        print("new value:", new_value)
        print("self.__fa:", self.__fa)
        print("getattr(self, '__fa'):", getattr(self, '__fa'))
        print("self.__dict__['__fa']:", self.__dict__['__fa'])

        assert self.__fa is not None


class Public:

    def __init__(self):
        print('\n############### PUBLIC ###############')
        print('Assign fa with None using setattr.')
        setattr(self, 'fa', None)


        print("Set fa to 42 using regular affectation.")
        new_value = 42
        self.fa = new_value

        print("\nPrint the new values. Expected to be all 42:")
        print("new value:", new_value)
        print("self.fa:", self.fa)
        print("getattr(self, 'fa'):", getattr(self, 'fa'))
        print("self.__dict__['fa']:", self.__dict__['fa'])

        assert self.__dict__['fa'] is not None  # this is expected
        assert getattr(self, 'fa') is not None  # this is expected


Private()
Public()

这种行为是预期的吗？为什么以及它的起源是什么？

Answer 1

这是预期的。在编译时执行的名称修饰不会影响字符串文字，因此您应该在 getattr、setattr 和 hasattr 中显式提供修饰的名称:

class Short:
    def __init__(self):
        self.__myatt = 42
        # print(getattr(self, '__myatt'))  # Fails with a NameError 
        print(getattr(self, '_{0.__qualname__}__myatt'.format(type(self))))  # Succeeds

现在打印出42。同样，要设置 setattr 和检查 hasattr，您需要提供损坏的名称。

这是在相关的 bug report 中提出的对于 hasattr 来说，是的，它被明确声明为预期行为。