[Python-ideas] __enter__ and __exit__ on generators

[Guido van Rossum]

Hm, it's probably just an oversight. Context managers and closable
generators were developed around the same time (PEP 342 and 343!) for
Python 2.5; it probably took a while before either of them became part
of the popular culture, so to speak.

On Tue, Jul 5, 2016 at 7:56 AM, Nathaniel Smith <njs at pobox.com> wrote:
> Hi all,
>
> This might be too trivial for python-ideas even, but... I recently stumbled
> slightly over the way that generators define a close method (the one that
> injects a GeneratorExit), but don't directly implement the context manager
> protocol. So if you want a generator's internal resources to be cleaned up
> promptly, you can't write
>
>   with genfunc() as it:
>       for obj in it:
>           ...
>
> but instead have to write
>
>   with contextlib.closing(genfunc()) as it:
>       for obj in it:
>           ...
>
> Is there any particular reason for this, or is it just an oversight?
>
> (Why is it important to close generators? Consider a generator that holds an
> open file descriptor:
>
>   def genfunc():
>       with open(...) as f:
>           for line in f:
>               yield ...
>
> Python these days very much encourages you to use a "with" statement for the
> file open, with ResourceWarnings and all that. But in this case, the file
> won't be closed until someone calls generator.close(), so the with statement
> above is only really effective if genfunc's caller also uses a "with"
> statement. Unfortunately, having the "with" statement inside genfunc *is*
> enough to unconditionally hide the ResourceWarning, even if it's otherwise
> ineffective, because the warning is triggered when file.__del__ calls
> file.__close__, and here we have generator.__del__ calling file.__close__.)
>
> -n
>
>
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-- 
--Guido van Rossum (python.org/~guido)