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

[Guido van Rossum]

> Yeah, I know. I'm not too proud of it but I was looking for instant 
> gratification...

The search for instant gratification probably ties a lot of the Python
community together...

> > Of course, since Jython has garbage collection, your finally clause
> > may be executed later than you had expected it, or not at all!  Are
> > you sure you want this?  
> 
> The same question applies to the __del__ method of any local variables 
> inside the suspended generator.  I tend to rely on the reference counting
> semantics of CPython in much of my code and I don't feel bad about it.

But __del__ is in essence asynchronous.  On the other hand,
try/finally is traditionally completely synchronous.  Adding a case
where a finally clause can execute asynchronously (or not at all,
if there is a global ref or cyclical garbage keeping the generator
alive) sounds like a breach of promise almost.

> > There are a few other places that invoke Python code in a dealloc
> > handler (__del__ invocations in classobject.c and typeobject.c).  They
> > do a more complicated dance with the reference count.  Can you check
> > that you are doing the right thing?
> 
> The __del__ method gets a reference to the object so it needs to be
> revived.  Generators are much simpler because the generator function does 
> not have any reference to the generator object.

But you still have to be careful with how you incref/decref -- see my
fatal error report in debug mode.

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