How to implement retrying a lock tidily in Python?

[Dave Angel]

  On 2:59 PM, tinnews at isbd.co.uk wrote:
> I'm writing some code that writes to a mbox file and want to retry
> locking the mbox file a few times before giving up.  I can't see a
> really tidy way to implement this.
>
> Currently I have something like:-
>
>      dest = mailbox.mbox(mbName, factory=None)
>
>      for tries in xrange(3):
>          try:
>              dest.lock()
>              #
>              #
>              # Do some stuff to the mbox file
>              #
>              dest.unlock()
>              break       # done what we need, carry on
>
>          except mailbox.ExternalClashError:
>              log("Destination locked, try " + str(tries))
>              time.sleep(1)
>              # and try again
>
> ... but this doesn't really work 'nicely' because the break after
> dest.unlock() takes me to the same place as running out of the number
> of tries in the for loop.  I need a way to handle the case where we
> run out of tries (and *haven't* done what we needed to do) separately
> from the case where it worked OK.
>
> I can see all sorts of messy ways to handle this with a flag of some
> sort but is there a proper elegant way of doing it?
>
>
The usual idiom for telling whether you ever finished a loop is to use 
the else: clause.  Else is only executed if break was never executed.

     for ....
           if something
               do...success
               break
           else
               do ...  one failure
               (continue)
     else:
           ...None of the loops succeeded

In your case you're using try/except, rather than if/else, but the outer 
mechanism still holds.

DaveA