[Python-Dev] PEP 409 update [was: PEP 409 (original) (raw)

Barry Warsaw barry at python.org
Fri Feb 3 18:29:11 CET 2012

On Feb 03, 2012, at 08:52 AM, Ethan Furman wrote:

Not sure I'll ever use it this way, but:

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 Ellipsis

In that context, I have to say that the last line, even if it were written

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.

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

Cheers, -Barry

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