RFC: Proposal: Deterministic Object Destruction

[Steven D'Aprano]

On Thu, 01 Mar 2018 16:26:47 -0800, ooomzay wrote:

>> >> When does the destination file get closed?
>> >
>> > When you execute:-
>> >
>> >    del dst
>> >
>> > or:-
>> >
>> >    dst = something_else
>> 
>> What if you don't?
> 
> Then the resource will remain open until your script exits at which
> point it is probably not very well defined exactly when or even if the
> destructor/__del__ will be called.
> 
> I.e. Don't do this! Did you have some realistic case in mind or are you
> just probing the behaviour?


If you're going to *require* the programmer to explicitly del the 
reference:

    f = open("file")
    text = f.read()
    del f

then you might as well require them to explicitly close the file:

    f = open("file")
    text = f.read()
    f.close()

which we know from many years experience is not satisfactory except for 
the simplest scripts that don't need to care about resource management.

That's the fatal flaw in RAII: for resources that you care about their 
*timely* release, the problem is that the lifespan of the resource may 
not be the same as the lifetime of the object. Especially for files, the 
problem is that the lifespan of resource (the time you are actually using 
it) may be significantly less than the lifespan of the object holding 
onto that resource. Since there's no way for the interpreter to know 
whether or not you have finished with the resource, you have a choice:

- close the resource yourself (either explicitly with file.close(), 
  or implicitly with a context manager);

- or keep the resource open indefinitely, until such eventual time
  that the object is garbage collected and the resource closed.




-- 
Steve