[Python-ideas] Automatic context managers

[Ronald Oussoren]

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,

Ronald