Bugs item #1121475, was opened at 2005-02-12 11:48 Message generated for change (Comment added) made by bcannon You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1121475&group_id=5470
Category: Python Library Group: Python 2.4 Status: Closed >Resolution: Wont Fix Priority: 5 Submitted By: S Joshua Swamidass (sswamida) Assigned to: Nobody/Anonymous (nobody) Summary: Decorated functions are unpickleable Initial Comment: The decorator feature renders functions impossible to pickle if you end up wrapping the function in either a trivial lambda or a callable class. This is a significant/artificial limitation of the decorator which should not exist and prevents decorators from being used for many of my purposes. ========================== Examples: ========================== >>> def dec(f): ... return lambda a: f(a) ... >>> @dec ... def func(a): ... return a ... >>> import pickle >>> pickle.dumps(func) Traceback (most recent call last): ... pickle.PicklingError: Can't pickle <function <lambda> at 0x40160ae4>: it's not found as __main__.<lambda> >>> class C: ... def __init__(self, f): ... self.f=f ... def __call__(self, a): ... return f(a) ... >>> def dec(f): ... return C(f) >>> @dec >>> def func(a): ... return a >>> import pickle >>> pickle.dumps(func) Traceback (most recent call last): ..... pickle.PicklingError: Can't pickle <function func at 0x40160a74>: it's not the same object as __main__.func ============================== I've found a syntacically ugly ways of working around the class wrapper bug that don't use decorators. Perhaps this could be used to create a decorator patch: ================================== >>> class C: ... def __init__(self, f): ... self.f=f ... def __call__(self, a): ... return f(a) ... >>> def _func(a): ... return a >>> func=C(_func) >>> >>> import pickle >>> pickle.dumps(func) (No error) ---------------------------------------------------------------------- >Comment By: Brett Cannon (bcannon) Date: 2005-02-16 21:10 Message: Logged In: YES user_id=357491 It would be nice if you could pickle decorated functions, but the way pickle works won't allow it unless the decorator just passes the function right along: >>> def dec(func): ... func.meta = "foo" ... return func ... >>> @dec ... def blah(): pass ... >>> pickle.dumps(blah) 'c__main__\nblah\np0\n.' So it is not a hard rule that decorated functions are not pickleable. It all depends on how the decorator messes with the function it is given. If you think the documentation for pickle is misleading then please submit a documentation patch to clarify that decorators could cause issues if they return the function wrapped. ---------------------------------------------------------------------- Comment By: S Joshua Swamidass (sswamida) Date: 2005-02-15 17:45 Message: Logged In: YES user_id=786138 Yes, everything you said is correct, though i think this is not ideal behavior. If we decorate a function at the top level of a module, shouldn't it be pickleable? If not, then as my original point was, decorated functions are not compatible with pickle. Is there any case you can give me where a decorated function is defined in the top level of a module and can be pickled? I can't think of one, but i could be wrong. ---------------------------------------------------------------------- Comment By: Brett Cannon (bcannon) Date: 2005-02-15 17:21 Message: Logged In: YES user_id=357491 If you read the pickle docs it says you can only pickle "functions defined at the top level of a module". The lambdas are you having your decorator return are not defined at the module level and thus do not meet that requirement. The same basic issue goes with your callable class. The function you are storing in self.f is not the same as was is defined at the module level since self.f is 'func' prior to wrapping while what 'func' has at the global level is an instance of 'C'. The reason your example works is because '_func` is defined at the module level and thus pickle can find it to pickle. Decorators skip the intermediate step of storing the undecorated function at the module level. It would be better to use the pickling protocol to define methods to allow your callable class to help with the pickling process. And just so you know, your class examples should be calling self.f instead of f. Closing as "won't fix". ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1121475&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
