[Python-Dev] Acquire/release functionality (original) (raw)

Moore, Paul Paul.Moore@atosorigin.com
Mon, 3 Feb 2003 14:31:39 -0000

Paul Moore <lists@morpheus.demon.co.uk> writes:

As far as the acquire/release case is concerned, I think that there is definitely a strong case for syntactic support for this. The equivalent idiom in C++ is "resource acquisition is initialisation" which, while not having syntax support, does rely on the existence of deterministic destructors, and on the ability to introduce a new scope at will. Python has neither of these, and the try...finally construct is verbose enough to make something better worth having.

In trying to draw a parallel between "with" and the C++ RAII idiom, I hit a funny, best illustrated by an example:

class autoclose(file):
    __exit__ =3D close

with f =3D autoclose("blah.txt", "r"):
    # Let's get sneaky
    g =3D f
    # Main code here

# File now gets closed
with g:
   # Who's a naughty boy then - g is a closed file
# And the g.__exit__() call goes boom here...

OK, it's contrived. The equivalent C++ risk is that of creating a = dangling pointer to a local variable which is deleted, which is also rare.

I don't know if this is a significant issue - after all, I can write code which closes a closed file object in normal Python. But I do think that we should remember the weird edge cases when discussing this sort of thing.

If nothing else, it strikes me as a mild argument in favour of allowing the assignment to be part of the "with" clause, just so that people don't get used to seeing a bare "with g" (which doesn't include any obvious clues that g needs auto-cleanup).

Paul.