setattr 和赋值不等同于私有属性

Question

使用 CPython 3.4.3 (GCC 5.3.1 20160406 (Red Hat 5.3.1-6))，我在私有属性上使用 setattr 和 getattr 时遇到了一个非常奇怪的行为：

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

尽管在断言语句之前做作，最后一段代码从未引发任何 AssertionError。但是，使用 myattr 而不是 __myatt 的相同代码会正确引发异常：

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

这是两种情况（私有与 public）的比较，带有注释和断言，表明 3 种访问方式中的 2 种未 returns 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

这是意料之中的。在编译时执行的名称修改不会影响字符串文字，因此您应该在 getattr、setattr 和 hasattr:

中明确提供修改后的名称

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，要检查 hasattr，您需要提供经过处理的名称。

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

setattr 和赋值不等同于私有属性

setattr and assignment are not equivalent for private attributes

python

python-3.x

attributes

cpython

setattr