On Thu, Jul 12, 2018 at 10:31 PM, Ed Kellett <e+python-l...@kellett.im> wrote: > On 2018-07-12 10:59, Steven D'Aprano wrote: >> On Thu, 12 Jul 2018 01:37:24 -0700, aleiphoenix wrote: >> >>> My question is, does except ... as ... create a new scope from outer >>> block, causing 'err' be hidden from outer scope? Is this intentional? >> >> No, it is not a new scope, and yes, it is intentional. It's a nasty hack, >> but a *necessary* nasty hack: when the except block exits, the "err" >> local variable (or whatever it happens to be called) is implicitly >> deleted. >> >> You can work around this by explicitly assigning to another local >> variable: >> >> try: >> ... >> except Exception as e: >> err = e # only "e" will be deleted when we exit the block >> >> >> This is necessary in Python 3 [...] > > "necessary" is debatable. When we have reference counting, general > garbage collection, *and* nasty hacks like this, one could be forgiven > for thinking Python has chosen the worst of all memory-management worlds.
I don't understand you. Are you saying that there are other memory management systems that are perfect, without any tradeoffs? Because the problem here is a reference cycle thus: >>> def f(): ... try: 1/0 ... except Exception as e: print(e is e.__traceback__.tb_frame.f_locals["e"]) ... >>> f() True Dealing with reference cycles is generally done *periodically* rather than immediately (CPython disposes of unreferenced objects immediately upon last deref). You can avoid having a dedicated cycle detection pass by using a mark-and-sweep GC, but that just means that *all* garbage is dealt with periodically rather than immediately. I can imagine having some sort of flag on every object that's involved in a cycle, and then doing a mini-GC every time any of those objects gets any dereferencing, but that would be ridiculously expensive. There are ALWAYS tradeoffs. How is Python's arrangement "the worst"? Personally, I think this would be better with an *actual* subscope. But apparently that's a ridiculously insane suggestion with no chance whatsoever of being accepted. Kinda like assignment expressions. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
