[Python-Dev] is this the fault of import_fresh_module or pickle?

[Eli Bendersky]

On Tue, Jan 8, 2013 at 8:37 AM, R. David Murray <rdmurray at bitdance.com>wrote:

> On Tue, 08 Jan 2013 17:05:50 +0100, Stefan Krah <stefan at bytereef.org>
> wrote:
> > Eli Bendersky <eliben at gmail.com> wrote:
> > > Everything works fine. However, if I add import_fresh_module for the C
> module:
> > >
> > > from test.support import import_fresh_module
> > > import pickle
> > > C = import_fresh_module('xml.etree.ElementTree',
> fresh=['_elementtree'])
> > > P = import_fresh_module('xml.etree.ElementTree',
> blocked=['_elementtree'])
> >
> > sys.modules still contains the C version at this point, so:
> >
> > sys.modules['xml.etree.ElementTree'] = P
> >
> >
> > > tb = P.TreeBuilder()
> > > print(pickle.dumps(tb))
> >
> > > This interaction only seems to happen with pickle. What's going on
> here? Can we
> > > somehow improve import_fresh_module to avoid this? Perhaps actually
> deleting
> > > previously imported modules with some special keyword flag?
> >
> > pickle always looks up sys.modules['xml.etree.ElementTree']. Perhaps we
> > could improve something, but this requirement is rather special;
> personally
> > I'm okay with switching sys.modules explicitly in the tests, because that
> > reminds me of what pickle does.
>
> Handling this case is why having a context-manager form of
> import_fresh_module was suggested earlier in this meta-thread.  At
> least, I think that would solve it, I haven't tried it :)
>

Would you mind extracting just this idea into this discussion so we can
focus on it here? I personally don't see how making import_fresh_module a
context manager will solve things, unless you add some extra functionality
to it? AFAIU it doesn't remove modules from sys.modules *before* importing,
at this point.

Eli
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