[Python-Dev] PEP 409 - final?

[Guido van Rossum]

Not a bad idea.

On Wed, Feb 1, 2012 at 12:53 PM, Terry Reedy <tjreedy at udel.edu> wrote:
> On 2/1/2012 3:07 PM, Guido van Rossum wrote:
>>
>> On Wed, Feb 1, 2012 at 10:48 AM, Ethan Furman<ethan at stoneleaf.us>  wrote:
>>>
>>> Guido van Rossum wrote:
>>>>
>>>>
>>>> Hm... Reading this draft, I like the idea of using "raise X from
>>>> None", but I still have one quibble. It seems the from clause sets
>>>> __cause__, and __cause__ can indicate three things: (1) print
>>>> __cause__ (explicitly set), (2) print __context__ (default), (3) print
>>>> neither (raise X from None). For (1), __cause__ must of course be a
>>>> traceback object.
>>>
>>>
>>>
>>> Actually, for (1) __cause__ is an exception object, not a traceback.
>>
>>
>> Ah, sorry. I'm not as detail-oriented as I was. :-)
>>
>>>> The PEP currently proposes to use two special
>>>> values: False for (2), None for (3). To me, this has a pretty strong
>>>> code smell, and I don't want this pattern to be enshrined in a PEP as
>>>> an example for all to follow. (And I also don't like "do as I say,
>>>> don't do as I do." :-)
>>>
>>>
>>>
>>> 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 sounds like you are asking for a special class
> __NoException__(BaseException) to use as the marker.
>
> --
> Terry Jan Reedy
>
>
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-- 
--Guido van Rossum (python.org/~guido)