[Python-ideas] Yield-From: Revamped expansion

Greg Ewing greg.ewing at canterbury.ac.nz
Sun Mar 22 09:23:11 CET 2009

Nick Coghlan wrote:

> I'd adjust the inner exception handlers to exploit the fact that
> SystemExit and GeneratorExit don't inherit from BaseException:

But then anything thrown in that didn't inherit from
Exception would bypass giving the subiterator a chance
to handle it, which doesn't seem right.

The more I think about this, the more I'm wondering
whether I shouldn't ever try to call close() on the
subiterator at all, and just rely on it to finalize
itself when it's deallocated.

That would solve all problems concerning when and if
close() calls should be made (the answer would be "never").

It would also avoid the problem of a partially exhausted
iterator that's still in use by something else getting
prematurely finalized, which is another thing that's been
bothering me.

Here's another expansion based on that idea. When we've
finished with the subiterator for whatever reason --
it raised StopIteration, something got thrown in, we
got closed ourselves, etc. -- we simply drop our reference
to it. If that causes it to be deallocated, it's
responsible for cleaning itself up however it sees fit.

     _i = iter(EXPR)
             _u = _i.next()
         except StopIteration, _e:
             _r = _e.value
             while 1:
                     _v = yield _u
                 except BaseException, _e:
                     _m = getattr(_i, 'throw', None)
                     if _m is not None:
                         _u = _m(_e)
                         if _v is None:
                             _u = _i.next()
                             _u = _i.send(_v)
                     except StopIteration, _e:
                         _r = _e.value
         del _i
     RESULT = _r

> I think Antoine and PJE are right that the PEP needs some more actual
> use cases though.

The examples I have are a bit big to put in the PEP
itself, but I can include some links.


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