Python，如何在作为重载方法的类的一部分的字典对象上使用 __setattr__ ？

Question

如下面的代码所示，为什么我不能使用 __setattr__ 在作为重载方法的类的一部分的字典上设置值？我预计 b.hello 不会存在。

class MyClass():

    datastore = {}

    def __init__(self):
        self.datastore = {}

    def __getattr__(self, key):
        return self.datastore[key]

    def __setattr__(self, key, value):
        self.datastore[key] = value

a = MyClass()
b = MyClass()

a.hello = "err"

print a.hello # err
print b.hello # err

Answer 1

b.hello 打印您的字符串“err”，因为 datastore 是类本身的属性，而不是类对象的属性。因此，当你在a中初始化它时，b也可以访问它。

因此，从类中删除 datastore = {}。

此外，来自 Python docs :

if __setattr__() wants to assign to an instance attribute, it should not simply execute self.name = value — this would cause a recursive call to itself. Instead, it should insert the value in the dictionary of instance attributes, e.g., self.__dict__[name] = value. For new-style classes, rather than accessing the instance dictionary, it should call the base class method with the same name, for example, object.__setattr__(self, name, value).

因此，将您的代码更改为:

class MyClass(object): # Use new style classes
    def __init__(self):
        object.__setattr__(self, 'datastore', {}) # This prevents infinite recursion when setting attributes

    def __getattr__(self, key):
        return self.datastore[key]

    def __setattr__(self, key, value):
        self.datastore[key] = value

a = MyClass()
b = MyClass()

a.hello = "err"

print a.hello # Works
print b.hello # Gives an error