[Python-Dev] Pickling of Enums

[Guido van Rossum]

I'm confused. Hasn't this all been decided by the PEP long ago?


On Tue, Feb 18, 2014 at 9:11 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 02/15/2014 11:01 AM, Serhiy Storchaka wrote:
>
>> How Enum items should be pickled, by value or by name?
>>
>> I think that Enum will be used to collect system-depending constants, so
>> the value of AddressFamily.AF_UNIX can be 1 on
>> one platform and 2 on other. If pickle enums by value, then pickled
>> AddressFamily.AF_INET on on platform can be
>> unpickled as AddressFamily.AF_UNIX on other platform. This looks weird
>> and contrary to the nature of enums.
>>
>
> There is one more wrinkle to pickling by name (it's actually still there
> in pickle by value, just more obvious in pickle by name) -- aliases.  It
> seems to me the most common scenario to having a name represent different
> values on different systems is when on system A they are different, but on
> system B they are the same:
>
> System A:
>
>   class SystemEnum(Enum):
>       value1 = 1
>       value2 = 2
>
> System B:
>
>   class SystemEnum(Enum):
>       value1 = 1
>       value2 = 1
>
> If you're on system B there is no way to pickle (by name or value) value2
> such that we get value2 back on system A.  The only way I know of to make
> that work would be to dispense with identity comparison, use the normal ==
> comparison, and have aliases actually be separate objects (we could still
> use singletons, but it would be one per name instead of the current one per
> value, and it would also be an implementation detail).
>
> Thoughts?
>
> --
> ~Ethan~
>
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-- 
--Guido van Rossum (python.org/~guido)
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