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).
--
bartc
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