[Python-ideas] Automatic context managers

[anatoly techtonik]

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.


>  >
> > I don't know why it was not implemented in the first place. Any ideas?
>
> It was implemented a long time ago. The with statement was added because
> relying on automatic resource cleanup by a destructor might clean up the
> resource too late (for example because the file object is referenced by a
> local variable in a frame that's referenced by an exception).


You're speaking about immediate garbage collection for file object in
CPython, which is a narrowing. I am still questioning about the whole
concept. Automatic content managers are useful for other cases and other
Python implementations.


> >
> > Depending on the answer to the above, the solution can be different. I
> assume that this was done for a reason (probably immediate garbage
> collection is too expensive), but confirmation is welcome. Meanwhile the
> solution can be implemented with auto context manager, which __exit__
> method is automatically called when all links to created object are lost.
> >
> > Difference from usual "with something" is that it is transparent to the
> user (leaves less details to worry about) and makes code more beautiful --
> in the example above the assignment is made inside setup(...) parameter
> assignment. An example with ordinary "with" statement would look like:
> >
> >     with open('README.txt') as readme:
> >       setup(
> >         ....
> >         long_description=readme.read(),
> >         ....
> >       )
>
> In python 3.3 and later you can use contexlib.ExitStack:
>
>     with contextlib.ExitStack() as stack:
>         setup(
>            ...
>            long_description =
> stack.enter_context(open('README.txt')).read(),
>            ...
>         )
>

It is no better than an additional indented with statement - both require a
change to the whole parent block.


> But for simple scripts like a setup.py I wouldn't worry too much about
> closing files later than expected.
>

It is just a real-world user story to backup the rationale.
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