On Fri, 4 Mar 2016 02:00 am, Random832 wrote: > On Thu, Mar 3, 2016, at 06:52, 刘琦帆 wrote: >> I have just saw PEP 255, and it says that >> >> "A yield statement is not allowed in the try clause of a try/finally >> construct. The difficulty is that there's no guarantee the generator >> will ever be resumed, hence no guarantee that the finally block will ever >> get executed; that's too much a violation of finally's purpose to bear." >> from https://www.python.org/dev/peps/pep-0255/ > > I'm not sure I understand this reasoning. Why not simply execute it in > __del__ if it hasn't been reached until then? AIUI that is what C# does.
I believe that under certain circumstances, __del__ may never be executed at all; it may be executed under rather perilous circumstances where the interpreter is already shutting down; and even if it is executed, it is not guaranteed to be executed at any particular time or in any particular order. In other words, cleaning up in __del__ may be non-deterministic, while the whole point of try...finally is that the finally block is executed in a deterministic fashion. In any case, PEP 255 is obsolete: that is no longer a limitation of yield. # Python 2.7 py> def gen(): ... try: ... yield 1 ... finally: ... yield 2 ... py> it = gen() py> next(it) 1 py> next(it) 2 -- Steven -- https://mail.python.org/mailman/listinfo/python-list
