On Monday, 5 March 2018 13:59:35 UTC, 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()
>
> > 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.
Ahah... I see now you are running it from a shell so the exception is staying
in scope. We just need to include normal exception handling in the example to
fix this:-
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():
try:
c = C()
c.dostuff()
except:
print("Boom!")
main()
--
https://mail.python.org/mailman/listinfo/python-list
