python del语句的魔术方法？

Question

Quick question, what is the magic method python calls when you say del object ? I know it's not __del__ and it isn't __delete__ , so what gets called if anything? If nothing is called, then how can I customize the events that occur when you delete an object?

---

Take a look at the following example:

class SingletonError(Exception):
    pass

class Singleton(type):

    def __new__(metacls, name, parents, kwargs):
        cls = super(Singleton, metacls).__new__(metacls, name, parents, kwargs)
        cls._instance = None
        return cls

    def __call__(cls, *args, **kwargs):
        if not cls._instance:
            inst = cls.__new__(cls, *args, **kwargs)
            inst.__init__(*args, **kwargs)
            cls._instance = inst
            return cls._instance
        else:
            raise SingletonError("Cannot initialize multiple singletons.")

class Logger(object, metaclass = Singleton):

    def __new__(cls, *logging_args, **logging_kwargs):
        self = super(Logger, cls).__new__(cls)
        logging_args_dict = {'log%i' % pos : i for pos, i in enumerate(logging_args, 1)}
        kwargs = dict(logging_args_dict, **logging_kwargs)
        self.__dict__ = kwargs
        return self

log = Logger()

When I delete log I would like it to set Logger._instance to None again, so you can reinitialize the singleton. How might I do this?

Answer 1

You could use a weak reference instead; as all references to the Logger() instances are cleaned up, the weak reference won't prevent the object from being reaped.

import gc
import weakref

def __call__(cls, *args, **kwargs):
    gc.collect()  # optional, clear existing weak references
    inst = cls._instance and cls._instance()  # de-reference the weakref
    if inst is None:
        inst = cls.__new__(cls, *args, **kwargs)
        inst.__init__(*args, **kwargs)
        cls._instance = weakref.ref(inst)
        return inst
    else:
        del inst  # clear local early
        raise SingletonError("Cannot initialize multiple singletons.")

cls._instance is now a weak reference object; you call it to retrieve the referenced object, and if it returns None the object is gone and you'll need to create a new one.

Just take into account that the deletion is not necessarily going to be instantaneous; garbage collection needs to run to reap the object once the last regular reference has been removed.

The gc.collect() call as the first line in the __call__ ensures that any weak refences are cleaned up before testing.