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()
> You keep insisting that this is an easy thing. > We keep pointing out
> that it isn't. Now you're proving that you haven't even attempted any
> of this.
Nonsense. But you have got a result I have never seen in many years
and I would like to get to the bottom of it.
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