[Python-Dev] Pickling instances of nested classes (original) (raw)
Samuele Pedroni pedronis at strakt.com
Thu Mar 31 21:57:19 CEST 2005
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Walter Dörwald wrote:
Samuele Pedroni wrote:
Walter Dörwald wrote:
[User cases for pickling instances of nested classes] So is this change wanted? useful? implementable with reasonable effort? Or just not worth it?
notice that in this cases often metaclasses are involved or could easely be, so if pickling would honor reduce or reduceex on metaclasses (which right now it doesn't treating their instances as normal classes) one could roll her own solution without the burden for the language of implementing pickling of nested classes in general, so I think that would make more sense, to add support to honor reduce/reduceex for metaclasses. Sorry, I don't understand: In most cases it can be possible to work around the nested classes problem by implementing custom pickling functionality (getstate/setstate/reduce/reduceex). But it is probably impossible to implement this once and for all in a common base class, because there's no way to find the real name of the nested class (or any other handle that makes it possible to retrieve the class from the module on unpickling). And having the full name of the class available would certainly help in debugging.
that's probably the only plus point but the names would be confusing wrt modules vs. classes.
My point was that enabling reduce hooks at the metaclass level has propably other interesting applications, is far less complicated than your proposal to implement, it does not further complicate the notion of what happens at class creation time, and indeed avoids the implementation costs (for all python impls) of your proposal and still allows fairly generic solutions to the problem at hand because the solution can be formulated at the metaclass level.
If pickle.py is patched along these lines [*] (strawman impl, not much tested but test_pickle.py still passes, needs further work to support reduce_ex and cPickle would need similar changes) then this example works:
class HierarchMeta(type): """metaclass such that inner classes know their outer class, with pickling support""" def new(cls, name, bases, dic): sub = [x for x in dic.values() if isinstance(x,HierarchMeta)] newtype = type.new(cls, name, bases, dic) for x in sub: x.outer = newtype return newtype
def reduce(cls): if hasattr(cls, 'outer'): return getattr, (cls.outer, cls.name) else: return cls.name
uses the HierarchMeta metaclass
class Elm: metaclass = HierarchMeta
def init(self, **stuff): self.dict.update(stuff)
def repr(self): return "<%s %s>" % (self.class.name, self.dict)
example
class X(Elm): class Y(Elm): pass
class Z(Elm): pass
import pickle
x = X(a=1) y = X.Y(b=2) z = X.Z(c=3)
xs = pickle.dumps(x) ys = pickle.dumps(y) zs = pickle.dumps(z)
print pickle.loads(xs) print pickle.loads(ys) print pickle.loads(zs)
pedronis$ python2.4 example.py <X {'a': 1}> <Y {'b': 2}> <Z {'c': 3}>
[*]: --- pickle.py.orig Wed Mar 30 20:37:14 2005 +++ pickle.py Thu Mar 31 21:09:41 2005 @@ -298,12 +298,19 @@ issc = issubclass(t, TypeType) except TypeError: # t is not a class (old Boost; see SF #502085) issc = 0
reduce = None if issc:
self.save_global(obj)return
for x in t.__mro__:if x is not object and '__reduce__' in x.__dict__:reduce = x.__dict__['__reduce__']breakelse:self.save_global(obj)return # Check copy_reg.dispatch_table
reduce = dispatch_table.get(t)
if not reduce:reduce = dispatch_table.get(t) if reduce: rv = reduce(obj) else:
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