Re: [Python-Dev] PEP 343 - Abstract Block Redux
Guido van Rossum wrote: > [...] But some of the claims from PEP 3XX seem to be incorrect now: Nick > claims that a with-statement can abstract an except clause, but that's > not the case; [...] Sorry for being a lurker, but can I try and expand this point. The options: - If we don't allow the except clause in the generator, the exception can't be examined there. - If we do allow the except clause we must (IMO) also allow the generator to suppress the exception. It would be surprising behaviour if an a caught exception was re-raised without an explicit raise statement. An argument: Despite the control-flow-macros-are-harmful discussion, I see nothing wrong with a block controller swallowing its block's exceptions because: - In most proposals it can raise its own exception in place of the block's exception anyway. - In the following example there is nothing surprising if controller() swallows block()'s exception: def block(): # do stuff raise E controller(block) Perhaps we don't want the block controller statement to have as much power over its block as controller() has over block() above. But handling an exception is not so radical is it? - Arnold. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 343 rewrite complete
Guido van Rossum wrote: > [Phillip J. Eby] >>* The transaction handler could also be written as: >> >> @with_template >> def transactional(db): >> db.begin() >> try: >> yield db >> except: >> db.rollback() >> else: >> db.commit() >> >>at least, if I understand it correctly. > > > Ah, of course. I've updated the PEP. > This template eats eats the exception, which will cause a RuntimeError in the proposed Wrapper, I think. A raise after rollback is needed. - Arnold ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 343 rewrite complete
Arnold deVos wrote:
>
> This template eats eats the exception, which will cause a RuntimeError
> in the proposed Wrapper, I think. A raise after rollback is needed.
Actually, the Wrapper as written in the PEP does not raise RuntimeError
if the generator catches a block's exception.
Shouldn't the relevant clause in the Wrapper go like this:
try:
self.gen.throw(type, value, traceback)
except type:
return
except StopIteration:
raise RuntimeError("generator caught exception")
else:
raise RuntimeError("generator didn't stop")
And the transaction template would go like this (re-raising the exception):
@with_template
def transactional(db):
db.begin()
try:
yield None
except:
db.rollback()
raise
else:
db.commit()
At least this is what I gleaned from the earlier threads. It means that
the template does not appear to supress an exception that it cannot
actually supress.
- Arnold
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Re: [Python-Dev] PEP 343 rewrite complete
Guido van Rossum wrote: > >[...] a generator doing cleanup depending on the >exception thrown (like the transactional() example below) can >*catch* the exception thrown if it wants to and doesn't have to >worry about re-raising it. I find this more convenient for the >generator writer. Against this was brought in that the >generator *appears* to suppress an exception that it cannot >suppress: the transactional() example would be more clear >according to this view if it re-raised the original exception >after the call to db.rollback(). [...] Of course, the explicit re-raise is only needed in a minority of use cases where the exception is caught. Two additional points in favour of this: - refactoring a naked try as a with + template is more direct and uniform across all use cases. - it is upwards compatible if the prohibition on templates suppressing exceptions is ever reconsidered. (Flow control macro discussion not withstanding.) - Arnold ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
