[Python-ideas] Automatic context managers

[anatoly techtonik]

On Fri, Apr 26, 2013 at 4:45 PM, Ronald Oussoren <ronaldoussoren at mac.com>wrote:

>
> On 26 Apr, 2013, at 15:22, anatoly techtonik <techtonik at gmail.com> wrote:
>
> > On Thu, Apr 25, 2013 at 8:46 AM, Ronald Oussoren <ronaldoussoren at mac.com>
> wrote:
> >
> > On 25 Apr, 2013, at 6:00, anatoly techtonik <techtonik at gmail.com> wrote:
> >
> > > On Wed, Apr 24, 2013 at 12:23 PM, Ronald Oussoren <
> ronaldoussoren at mac.com> wrote:
> > >
> > >
> > >
> > >
> > > On 24 Apr, 2013, at 10:59, anatoly techtonik <techtonik at gmail.com>
> wrote:
> > >
> > > > Long time no see, all. :P
> > > >
> > > > PySide Qt binding have an interesting property - when you create
> widgets, you need to assign them to variables. When such variable is lost,
> object is immediately destroyed.
> > > >
> > > > I often use this one-shot code in setup.py:
> > > >    ...
> > > >    long_description = open('README.txt').read(),
> > > >    ....
> > > >
> > > > Which probably leaves the README.txt file open until the setup.py
> exits. So, the idea is to close the file as soon as the variable is lost.
> > >
> > > The file is automaticly closed as soon as the file object is garbage
> collected. In your example CPython would currently collect at the end of
> the read call (unles there is an exception) because of the reference
> counting garbage collector, but other implementations have other garbage
> collectors and can collect the file object (much) later.
> > >
> > > Right. Automatic context manager proposal brings this mechanism from
> garbage collection implementation level to language definition level.
> >
> > What proposal?   What you appear to propose is either that
> implementations must use a reference counting collector (more or less
> ensuring that the file will be closed after the call to read in your
> example), or that the exit part of the context protocol is run whenever an
> object is going out of scope.
> >
> > The proposal is fully illustrated by the user story above
>
> Right, there is no proposal, only vague handwaving.  I haven't seen
> anything yet that wouldn't require the use of refcounting (the file is
> closed as soon as the last reference to the file object goes away), or some
> serious magic (when you want the file object to be closed even when read
> raises an exeption).
>
> When you want to propose something you need to do some work yourself. That
> doesn't mean you have to provide a patch, but you do need to specify your
> proposal detailed enough to understand it without trying to second guess
> you.
>
> The batteries of my crystal ball ran out,
>

Ok. The proposal is patch Python to be able to write:

   boolean = open(resource).use()

Instead of:

   boolean = None
   with open(resource) as tempvar:
       boolean = tempvar.use()

That's it. I am not pretending I know how to implement it. I just expressed
my opinion that this might be possible, because PySide seems to do this
somehow.
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