[Python-Dev] doctest and pickle

[PJ Eby]

On Fri, Jun 7, 2013 at 1:54 PM, Mark Janssen <dreamingforward at gmail.com> wrote:
> On Fri, Jun 7, 2013 at 10:50 AM, Mark Janssen <dreamingforward at gmail.com> wrote:
>>>     >>> from pickle import dumps, loads
>>>     >>> Fruit.tomato is loads(dumps(Fruit.tomato))
>>>     True
>>
>> Why are you using is here instead of ==?  You're making a circular
>> loop using "is"
>
> I should add that when you're serializing with pickle and then
> reloading, the objects should be seen as "essentially equivalent".
> This means that they are either byte-by-byte equivalent (not sure
> actually if Python actually guarantees this), or every element would
> still compare equal and that is what matters.

For global objects such as functions and classes -- and singletons
such as None, Ellipsis, True, and False -- pickling and unpickling is
actually supposed to retain the "is" relationship as well.

I don't know if enums *actually* preserve this invariant, but my
default expectation of the One Obvious Way would be that enums, being
uniquely-named objects that know their name and container, should be
considered global objects in the same fashion as classes and
functions, *and* that as singletons, they'd also be treated in the
same way as None, Ellipsis, etc.  That is, there are two independent
precedents for objects like that preserving "is" upon pickling and
unpickling.

(As another precedent, my own SymbolType library (available on PyPI)
preserves the "is"-ness of its named symbol objects upon pickling and
unpickling.)