[Python-Dev] PEP 409 - final?

[Terry Reedy]

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