[Python-ideas] Enum and serialization

[Guido van Rossum]

So you want to make print(<intenum>) output '42' instead of
'THE_ANSWER'? I am strongly against that change that.

On Thu, Jul 18, 2013 at 10:11 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 19 July 2013 04:24, Ethan Furman <ethan at stoneleaf.us> wrote:
>> On 07/18/2013 11:13 AM, Giampaolo Rodola' wrote:
>>>
>>> I just received this bug report in psutil:
>>> https://code.google.com/p/psutil/issues/detail?id=408
>>>
>>> Long story short, in psutil I have defined a constant type:
>>> http://code.activestate.com/recipes/577984-named-constant-type/
>>> https://code.google.com/p/psutil/source/browse/psutil/_common.py#31
>>> ...behaving along these lines:
>>>
>>>>>> FOO = constant(0, 'FOO')
>>>>>> FOO
>>>
>>> 0
>>>>>>
>>>>>> str(FOO)
>>>
>>> 'FOO'
>>>
>>> It's a nasty problem and I'm still not sure how to solve it in psutil
>>> but this got me thinking about PEP 435 (Adding an Enum type to the
>>> Python standard library) since the two things are kind of related.
>>> I haven't properly gone through the PEP yet (I will) but I notice it
>>> doesn't talk about serialization.
>>> Has it been considered?
>>
>>
>> See http://bugs.python.org/issue18264.
>>
>> The basic issue is that json only knows how to serialize built-in types.  As
>> soon as we build on that, json barfs.
>>
>> One way around that is to write your own json handler that knows about your
>> custom types.
>>
>> For the 3.4 stdlib there are two proposals on the table:
>>
>>   1) for IntEnum and FloatEnum cast the member to int or float, then
>> procede;
>>
>>   2) for any Enum, extract the value and proceed.
>
> 3) Only change repr (not str) in IntEnum
>
> I know Guido doesn't like it, but I still think the backwards
> compatibility risk is too high to use them to replace constants in the
> standard library if they change the output of __str__. Debugging only
> needs repr, and we can make sure stdlib error messages use repr, too.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> http://mail.python.org/mailman/listinfo/python-ideas



-- 
--Guido van Rossum (python.org/~guido)