On Monday, 5 March 2018 15:17:13 UTC, bartc wrote:
> On 05/03/2018 13:58, Ooomzay wrote:
> > On Monday, 5 March 2018 11:24:37 UTC, Chris Angelico wrote:
> >> On Mon, Mar 5, 2018 at 10:09 PM, Ooomzay wrote:
> >>> Here is an example of a composite resource using RAII:-
> >>>
> >>> class RAIIFileAccess():
> >>> def __init__(self, fname):
> >>> print("%s Opened" % fname)
> >>> def __del__(self):
> >>> print("%s Closed" % fname)
> >>>
> >>> class A():
> >>> def __init__(self):
> >>> self.res = RAIIFileAccess("a")
> >>>
> >>> class B():
> >>> def __init__(self):
> >>> self.res = RAIIFileAccess("b")
> >>>
> >>> class C():
> >>> def __init__(self):
> >>> self.a = A()
> >>> self.b = B()
> >>>
> >>> def main():
> >>> c = C()
> >>>
> >>> Under this PEP this is all that is needed to guarantee that the files "a"
> >>> and "b" are closed on exit from main or after any exception has been
> >>> handled.
> >>
> >> Okay. And all your PEP needs is for reference count semantics, right?
> >> Okay. I'm going to run this in CPython, with reference semantics. You
> >> guarantee that those files will be closed after an exception is
> >> handled? Right.
> >>
> >>>>> def main():
> >> ... c = C()
> >> ... c.do_stuff()
> >> ...
> >>>>> main()
> >> a Opened
> >> b Opened
> >> Traceback (most recent call last):
> >> File "<stdin>", line 1, in <module>
> >> File "<stdin>", line 3, in main
> >> AttributeError: 'C' object has no attribute 'do_stuff'
> >>>>>
> >>
> >> Uhh.... I'm not seeing any messages about the files getting closed.
> >
> > Then that is indeed a challenge. From CPython back in 2.6 days up to
> > Python36-32 what I see is:-
> >
> > a Opened
> > b Opened
> > Traceback (most recent call last):
> > ...
> > AttributeError: 'C' object has no attribute 'dostuff'
> > a Closed
> > b Closed
> >
> >> Maybe exceptions aren't as easy to handle as you think?
> >
> > Well there is a general issue with exceptions owing to the ease
> > with which one can create cycles that may catch out newbs. But
> > that is not the case here.
> >
> >> Or maybe you
> >> just haven't tried any of this (which is obvious from the bug in your
> >> code
> >
> > Or maybe I just made a typo when simplifying my test case and failed to
> > retest?
> >
> > Here is my fixed case, if someone else could try it in CPython and report
> > back that would be interesting:-
> >
> > class RAIIFileAccess():
> > def __init__(self, fname):
> > print("%s Opened" % fname)
> > self.fname = fname
> >
> > def __del__(self):
> > print("%s Closed" % self.fname)
> >
> > class A():
> > def __init__(self):
> > self.res = RAIIFileAccess("a")
> >
> > class B():
> > def __init__(self):
> > self.res = RAIIFileAccess("b")
> >
> > class C():
> > def __init__(self):
> > self.a = A()
> > self.b = B()
> >
> > def main():
> > c = C()
> > c.dostuff()
> >
> > main()
>
> I get A and B closed messages when running on CPython 2.7 and 3.6, with
> the code run from a .py file. (I never use interactive mode.)
>
> But not when running on a PyPy version of 2.7 (however that is not CPython).
Thanks bartc, I have made the example more complete by adding an exception
scope - this means it works as designed - in any context. See my reply to Chris.
We do not expect this to work in PyPy.
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