pickle question: sequencing of operations

[Ian Kelly]

On Wed, May 9, 2012 at 2:34 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Wed, May 9, 2012 at 1:39 PM, Russell E. Owen <rowen at uw.edu> wrote:
>> I was wondering. I override __new__ (and __init__) to print messages and
>> was quite surprised to only see __new__being called when the object was
>> first created, not when it was being unpickled. But maybe there's
>> something funny about my override that caused unpickle to ignore it and
>> use the default version. I hope so. I can't see how the object could be
>> constructed during unpickle without calling __new__. But that's one
>> reason I was curious about the unpickling sequence of operations.
>
> You're probably pickling with the default protocol.  Unpickling calls
> an overridden __new__ method only if the pickle protocol is at least
> 2.  Using protocol 0 or 1, new-style class instances are constructed
> with the base object.__new__ instead.

BTW, in case you're wondering where all this is documented, pull up
PEP 307 and read the sections "Case 2" and Case 3".

Cheers,
Ian