[Python-Dev] patch: try/finally in generators

[Guido van Rossum]

[Tim]
> Note that we already allow try/finally in generators.  The only
> prohibition is against having a yield stmt in the try clause of a
> try/finally construct (YINTCOATFC).
> 
[Neil]
> > then they can cause uncollectible garbage.  It's not a show
> > stopper but something else to take into consideration.
> 
> I'm concerned about semantic clarity.  A "finally" block is supposed
> to get executed upon leaving its associated "try" block.  A yield
> stmt doesn't leave the try block in that sense, so there's no
> justification for executing the finally block unless the generator
> is resumed, and the try block is exited "for real" via some other
> means (a return, an exception, or falling off the end of the try
> block).  We could have allowed YINTCOATFC under those rules with
> clarity, but it would have been a great surprise then that the
> finally clause may never get executed at all.  Better to outlaw it
> than that (or, as the PEP says, that would be "too much a violation
> of finally's purpose to bear").
> 
> Making up new control flow out of thin air upon destructing a
> generator ("OK, let's pretend that the generator was actually
> resumed in that case, and also pretend that a return statement
> immediately followed the yield") is plainly a hack; and because it's
> still possible then that the finally clause may never get executed
> at all (because it's possible to create an uncollectible generator),
> it's too much a violation of finally's purpose to bear even so.
> 
> When I've needed resource-cleanup in a generator, I've made the
> generator a method of a class, and put the resources in instance
> variables.  Then they're easy to clean up at will (even via a
> __del__ method, if need be; but the uncertainty about when and
> whether __del__ methods get called is already well-known, and I
> don't want to extend that fuzziness to 'finally' clauses too -- we
> left those reliable against anything short of a system crash, and
> IMO it's important to keep them that bulletproof).

I hope that Oren will withdraw his patch based upon this explanation.

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