I have a class like this:
class Something(object):
def __init__(self):
self._thing_id
self._cached_thing
@property
def thing(self):
if self._cached_thing:
return self._cached_thing
return Thing.objects.get(id=self._thing_id)
When pickling objects like this, I'd like to prevent pickling of the _cached_thing field, as it's volatile and a specifically in-memory-only implementation.
Is there a way to suggest to Pickle that I only want a subset of my fields to be pickled?
Pickle can be customized in three ways, as described in the docs.
__getstate__ and __setstate__ methods.__getnewargs__/__getnewargs_ex__ (and a constructor that takes those args).__reduce__ (and a function to give to __reduce__ to reverse it).The first is usually the simplest:
class Something(object):
def __init__(self):
self._thing_id
self._cached_thing
def __getstate__(self):
return self._thing_id
def __setstate__(self, thing_id):
self._thing_id = thing_id
# etc.
If you want something more generic, that will pickle all values (including those set by a subclass, or dynamically after creation, etc.) except your blacklist, note that the default is "the instance's __dict__ is pickled", so just filter that:
_blacklist = ['_cached_thing']
def __getstate__(self):
return {k: v for k, v in self.__dict__.items() if k not in self._blacklist}
def __setstate__(self, state):
self.__dict__.update(state)
And please see gnibbler's comment on the question: if you're doing something generic, you should seriously consider coming up with some kind of naming convention instead of putting a blacklist in each class. Any reader who knows or learns the convention will immediately know which properties are "cache" values rather than part of the "real" value, it'll be more obvious how things work, there's less work for you to do in each class, and fewer places to screw things up with a typo…