Rhodri James schreef op 27/02/2019 om 15:18:
On 27/02/2019 06:56, Marko Rauhamaa wrote:
Alan Bawden <a...@csail.mit.edu>:
But I appreciate that that isn't the true question that you wanted to ask!
You are wondering if a Python implementation is _permitted_ to treat the
code you wrote _as if_ you had written:
def fun():
f = open("lock")
flock.flock(f, fcntl.LOCK_EX)
del f
do_stuff()
sys.exit(0)
which deletes the variable f from the local environment at a point where it
will never be used again. (Which could cause the lock to be released
before do_stuff is even called.)
This is an interesting question, and one that any garbage collected
language should probably address somehow.
Interesting indeed. My gut instinct was "Hell, no!", but as Marko
points out C optimisers do exactly that, and I don't find it
particularly surprising in practice. I don't think that there is
anything in the docs that says a compliant implementation couldn't
delete variables early. The nearest you get is repeated reminders that
you can't make assumptions about when destructors will be run.
Aren't we overthinking this?
I think it's pretty clear that a variable is never deleted before it
goes out of scope. A quick search in the documentation points me to
(https://docs.python.org/3/reference/datamodel.html#objects-values-and-types):
"Objects are never explicitly destroyed; however, when they become
unreachable they may be garbage-collected. An implementation is allowed
to postpone garbage collection or omit it altogether — it is a matter of
implementation quality how garbage collection is implemented, *as long
as no objects are collected that are still reachable*." (emphasis mine)
In the original example (without del), f is reachable everywhere in the
function after the initial binding, so it can not be deleted.
Comparisons with C on this point don't seem very relevant: C doesn't
have destructors or garbage collection. I don't even see what the C
equivalent for "del f" could be. You could perhaps compare with C++,
where destructors are also not called before the object goes out of
scope (a difference is that in C++ the destructor explicitly always is
called at that moment).
Regards,
Roel
--
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friend, an acquaintance, or a stranger."
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Roel Schroeven
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