__init__.py 文件源码

def __reduce__(self):
        """
        Pickling should return the model._meta.fields instance of the field,
        not a new copy of that field. So, we use the app registry to load the
        model and then the field back.
        """
        if not hasattr(self, 'model'):
            # Fields are sometimes used without attaching them to models (for
            # example in aggregation). In this case give back a plain field
            # instance. The code below will create a new empty instance of
            # class self.__class__, then update its dict with self.__dict__
            # values - so, this is very close to normal pickle.
            state = self.__dict__.copy()
            # The _get_default cached_property can't be pickled due to lambda
            # usage.
            state.pop('_get_default', None)
            return _empty, (self.__class__,), state
        return _load_field, (self.model._meta.app_label, self.model._meta.object_name,
                             self.name)