On Wed, May 27, 2009 at 12:17:36PM +0200, Hanno Schlichting wrote: > Chris Withers wrote: > > Hanno Schlichting wrote: > >> They are incredibly expensive to unpickle since all > >> the DWIM magic in their __init__ get called each time, though. > > > > How come? Unpickling doesn't call __init__ and I don't see why the DWIM > > magic would be needed anyway, since everything has already been parsed. > > How would a new instance of a class be constructed without calling the > init or new?
You're cleverly changing the question ;-) *Most* objects are unpickled by calling __new__ followed by __setstate__, but without calling __init__. Chris is therefore understandably surprised. Old-style classes that define __getinitargs__ will get their __init__ called during unpickling, though, and DateTime is such a class. You could also cause the __init__ to be called by defining __reduce__ or __reduce_ex__ appropriately. This is all described in great detail at http://python.org/doc/current/library/pickle.html#pickle-protocol >Look at the _instantiate method in pickle.py, when it does: > > value = klass(*args) > > What happens on unpickling is that a new DateTime instance representing > "just now" is generated and then that instance is updated with the > values from the pickle. Marius Gedminas -- Every nonempty totally-disconnected perfect compact metric space is homeomorphic to the Cantor set.
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