On Aug 8, 2011, at 4:44 PM, Chris McDonough wrote:
> Kudos to whomever turned the "transaction" package's transaction manager into
> a context manager and was thoughtful enough to provide the "attempts" method
> (which returns a separate context manager, wrapping the txn context manager,
> and retries some number of configurable times).
> This makes writing code that integrates with a web system that commits
> or aborts as easy as:
>
> import transaction
>
> for attempt in transaction.attempts(attempts):
> with attempt as t:
> response = handler(request)
>
> if t.isDoomed():
> t.abort()
>
> if commit_veto is not None:
> veto = commit_veto(request, response)
> veto and t.abort()
>
> return response
>
> Very nice.
>
> - C
Yes, this is *very* convenient and I like to use it, but beware of the bug
(it's not affecting you above because of the return).
According to the documentation, it's intended as a replacement for:
for i in range(3):
try:
with transaction:
... some something ...
except SomeTransientException:
continue
else:
break
So, presumably,
for attempt in transanction.attempts(retries):
with attempt as tx:
do_something()
should break after the *first* successful commit.
It doesn't. Instead, it commits do_something() ``retries`` times which is most
probably not intended.
See https://bugs.launchpad.net/transaction/+bug/724332
The patch attached to the bug report is just a test that proves that behavior
is unexpected and gives a workaround.
The workaround is simple enough and I prefer to use transaction.attempts
convenience rather than the typical try-except code above.
Still, we really should find time to fix this in the transaction manager code
so that it behaves as expected. (Or change the documentation and tests).
Regards,
Zvezdan
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