[Python-Dev] doctest and pickle (original) (raw)

PJ Eby pje at telecommunity.com
Fri Jun 7 20:30:39 CEST 2013

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.)

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