[Python-Dev] Another PEP 343 contextmanager glitch (original) (raw)

Guido van Rossum guido at python.org
Sat Mar 25 05:13:42 CET 2006

On 3/24/06, Phillip J. Eby <pje at telecommunity.com> wrote:

At 04:34 PM 3/24/2006 -0800, Guido van Rossum wrote: >You could probably tell the difference by comparing the exception it >raises to the exception you pass it.

That's what I changed @contextmanager to do, because it's the only place where you have a good reason to re-raise (i.e., the generator re-raises in order to request propagation). Thus, this inspection code should only have to appear in one place: @contextmanager, rather than it having to be in every other context that needs to tell the difference.

Ah. After reading what you checked in I understand it better. The issue is the slight impedance mismatch between throw() and exit(): throw() always has to re-raise the exception, because returning a value already has a different meaning (the generator yielded again). That's what I originally implemented for exit() after I realized the need for a way to swallow exceptions, but then after reasonable opposition that this would make writing a "vanilla" exit() a bit tricky in a way that encourages buggy code, I changed it to returning True to swallow the exception, and returning False or re-raising to re-raise. (returning in any way also meant to continue a non-local goto.)

I think that the code you added:

could use a comment explaining what's going on; the difference between returning None and re-raising the exception is really quite subtle here, and only ever matters if something else besides the with-statement framework is calling exit. (I would add this distinction to the PEP too.)

By the way, I just got approval earlier this week to work on various Python 2.5 tasks as a "20% project" at OSAF, so today's my day this week to try to burn through as many outstanding Python 2.5 issues as possible. Hence all my activity on PEPs 342 and 343. My current rough plans for wsgiref, setuptools and the rest can be found at:

http://peak.telecommunity.com/DevCenter/Python25 if anybody's interested.

Great news! I'm glad you're planning to finish up the PEP 302 stuff.

I'd like to see some discussion of your proposed change to .pth file processing; this could cause some surprises and will slow down all Python's start-up time due to the necessity to list every directory on sys.path. This is IMO a bad thing; there must be nothing to keep people from writing tiny scripts in Python, and slow startup time is an inhibitor there. (If you don't believe this could be a problem, try it with a long path containing several directories on NFS. This is the standard setup of Google developer workstations, and it gives one a different outlook on life.)

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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