[Python-Dev] with-statement heads-up (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Feb 28 23:29:16 CET 2006

At 04:01 PM 2/28/2006, Guido van Rossum wrote:

On 2/28/06, Nick Coghlan <ncoghlan at gmail.com> wrote: > Guido van Rossum wrote: > > I just realized that there's a bug in the with-statement as currently > > checked in. exit is supposed to re-raise the exception if there > > was one; if it returns normally, the finally clause is NOT to re-raise > > it. The fix is relatively simple (I believe) but requires updating > > lots of unit tests. It'll be a while. > > So does that mean with statements will be able to suppress exceptions now? > (If I'm reading the PEP changes right it does, but I haven't finished my > coffee yet. . .)

Yes. And unless there are peasants at the gate with pitchforks etc. it will stay that way. :-)

Notice that these semantics break some of the PEP examples. For example, the 'locked' and 'nested' classes now suppress exceptions, and it took a non-trivial study of their code to determine this. This seems to suggest that making suppression the default behavior is a bad idea.

I was originally on the side of allowing suppression, but I wanted it to be done by explicitly returning some non-None value, so that suppression would not be the default, implicit behavior. I think I'd prefer not to be able to suppress the errors, than to have errors pass silently unless explicitly re-raised! I don't see a problem with having e.g. exit have to return a flag to suppress the exception; it wouldn't need to change how @contextmanager functions are written. (Implicit suppression is only a problem for people writing exit methods, in other words; all your reasoning about @contextmanager generators is valid, IMO.)

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