[Python-Dev] PEP 409 update [was: PEP 409 - final?]

[Ethan Furman]

Barry Warsaw wrote:
>     raise e from ...
> 
> is certainly cute, but not very informative.  Triple-dots will be confusing
> and difficult to read in documentation and code, and Ellipsis has no logical
> connection to the purpose of this PEP.  So while I'm +1 on everything else in
> the PEP, I'm -1 on this particular decision.
> 
> One of the alternatives states:
> 
>     Create a special exception class, __NoException__.
> 
>     Rejected as possibly confusing, possibly being mistakenly raised by users,
>     and not being a truly unique value as None, True, and False are.
> 
> I think this should be revisited.  First, `__NoException__` doesn't need to be
> an exception class.  Ellipsis isn't so this doesn't need to be either.  I have
> no problem adding a new non-exception derived singleton to mark this.  And
> while __NoException__ may be a little confusing, something like __NoCause__
> reads great and can't be mistaken for a raiseable exception.

The problem I have with names like __NoException__, __NoCause__, 
__NoWhatever__ is that is sounds a lot like None -- in otherwords, like 
there won't be any chaining.


> So your example would then be:
> 
> try:
>    try:
>      raise IndexError()
>    except:
>      raise CustomError() from None
> except CustomError as e:
>    # nevermind, let's see the whole thing after all
>    raise e from __NoCause__

If we do switch from Ellipsis to something else I think a name like 
__Chain__ would be more appropriate.  Or __ExcChain__.

   raise e from __ExcChain__

Less cute, but probably less confusing.

~Ethan~