python - deepcopy 不尊重元类

Question

我有一个类，根据设计，它必须遵循单例 模式。所以我继续使用 元类 实现它。一切都很好，直到报告了一个错误，总而言之，该错误说 deepcopy-ied 实例的我的 singleton 类不是相同的实例。

我可以通过继承 singleton 基类来绕过这个错误，但我宁愿不这样做，原因在 this question 中指出。 .

此问题的工作示例如下所示:

class SingletonMeta(type):
    def __init__(cls, name, bases, dict):
        super(SingletonMeta, cls).__init__(name, bases, dict)
        cls.instance = None 
    def __call__(cls,*args,**kw):
        print "SingletonMeta __call__ was called"
        if cls.instance is None:
            cls.instance = super(SingletonMeta, cls).__call__(*args, **kw)
        return cls.instance

class MyClass1(object):
    __metaclass__ = SingletonMeta

class SingletonBase(object):
    _instance = None
    def __new__(class_, *args, **kwargs):
        print "SingletonBase __new__ was called"
        if not isinstance(class_._instance, class_):
            class_._instance = object.__new__(class_, *args, **kwargs)
        return class_._instance

class MyClass2(SingletonBase):
  pass

from copy import deepcopy as dcp

mm1 = MyClass1()
mm2 = dcp(mm1)
print "mm1 is mm2:", mm1 is mm2

mb1 = MyClass2()
mb2 = dcp(mb1)
print "mb1 is mb2:", mb1 is mb2

输出:

SingletonMeta __call__ was called
mm1 is mm2: False
SingletonBase __new__ was called
SingletonBase __new__ was called
mb1 is mb2: True

你能给我一些关于如何解决这个问题的建议吗？我正在运行 Python 2.7.X

Answer 1

copy 模块上的文档是这样说的:

In order for a class to define its own copy implementation, it can define special methods __copy__() and __deepcopy__(). [...] The latter is called to implement the deep copy operation; it is passed one argument, the memo dictionary. [...]

因此，如果您将这些声明为返回 self，那应该可以解决问题。