Moving towards Python 3.0 (was Re: [Python-Dev] Speed up functioncalls)

[Glyph Lefkowitz]

On Mon, 2005-01-31 at 08:51 -0800, Michael Chermside wrote:

> However, remember that changing away from reference counting is a change
> to the semantics of CPython. Right now, people can (and often do) assume
> that objects which don't participate in a reference loop are collected
> as soon as they go out of scope. They write code that depends on
> this... idioms like:
> 
>     >>> text_of_file = open(file_name, 'r').read()
> 
> Perhaps such idioms aren't a good practice (they'd fail in Jython or
> in IronPython), but they ARE common. So we shouldn't stop using
> reference counting unless we can demonstrate that the alternative is
> clearly better. Of course, we'd also need to devise a way for extensions
> to cooperate (which is a problem Jython, at least, doesn't face).

I agree that the issue is highly subtle, but this reason strikes me as
kind of bogus.  The problem here is not that the semantics are really
different, but that Python doesn't treat file descriptors as an
allocatable resource, and therefore doesn't trigger the GC when they are
exhausted.

As it stands, this idiom works most of the time, and if an EMFILE errno
triggered the GC, it would always work.

Obviously this would be difficult to implement pervasively, but maybe it
should be a guideline for alternative implementations to follow so as
not to fall into situations where tricks like this one, which are
perfectly valid both semantically and in regular python, would fail due
to an interaction with the OS...?