[Python-Dev] PEP 409 - final?

[Guido van Rossum]

On Wed, Feb 1, 2012 at 12:55 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
> Guido van Rossum wrote:
>>
>> On Wed, Feb 1, 2012 at 10:48 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>>>
>>> My apologies for my ignorance, but is the code smell because both False
>>> and
>>> None evaluate to bool(False)?
>>
>>
>> That's part of it, but the other part is that the type of __context__
>> is now truly dynamic. I often *think* of variables as having some
>> static type, e.g. "integer" or "Foo instance", and for most Foo
>> instances I consider None an acceptable value (since that's how
>> pointer types work in most static languages). But the type of
>> __context__ you're proposing is now a union of exception and bool,
>> except that the bool can only be False.
>>
>> It seems you really need a marker object. I'd be fine with using some
>> other opaque marker -- IMO that's much better than using False but
>> disallowing True.
>
>
> So for __cause__ we need three values:
>
>  1) Not set special value (prints __context__ if present)
>
>  2) Some exception (print instead of __context__)
>
>  3) Ignore __context__ special value (and stop following the
>     __context__ chain)
>
> For (3) we're hoping for None, for (2) we have an actual exception, and for
> (1) -- hmmm.
>
> It seems like a stretch, but we could do (looking at both __context__ and
> __cause__):
>
>                   __context__          __cause__
>
> raise              None                 False [1]
>
> reraise            previous             True  [2]
>
> reraise from       previous             None [3] | exception
>
> [1] False means non-chained exception
> [2] True means chained exception
> [3] None means chained exception, but by default we do not print
>    nor follow the chain
>
> The downside to this is that effectively either False and True mean the same
> thing, i.e. try to follow the __context__ chain, or False and None mean the
> same thing, i.e. don't bother trying to follow the __context__ chain because
> it either doesn't exist or is being suppressed.
>
> Feels like a bunch of complexity for marginal value.  As you were saying,
> some other object to replace both False and True in the above table would be
> ideal.

So what did you think of Terry Reedy's idea of using a special exception class?

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