Question

使用CPython 3.4.3（GCC 5.3.1 20160406（Red Hat 5.3.1-6）），在私有属性上使用setattr和getattr时遇到一种非常奇怪的行为：

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

最后一个代码永远不会引发任何AssertionError，尽管在断言语句之前有效。但是，使用myattr代替__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()

以下是两种情况（私有与公共）的比较，带有注释和断言，表明3种访问方式中的2种不返回预期值：

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()

预计会出现这种情况吗？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改会使字符串文字保持不变，因此您应该在getattr，setattr和hasattr中明确提供损坏的名称：

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，您需要提供损坏的名称。

这是hasattr对main :: IO ()提出的，是的，是明确表示的预期行为。

私有属性的setattr和赋值不相等

1 个答案: