[Python-Dev] Strange artifacts with PEP 3121 and monkey-patching sys.modules (in csv, ElementTree and others)

Eli Bendersky eliben at gmail.com
Sun Aug 11 15:03:19 CEST 2013

On Sun, Aug 11, 2013 at 2:58 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

> On Sat, 10 Aug 2013 18:06:02 -0700
> Eli Bendersky <eliben at gmail.com> wrote:
> > This solution has problems. For example, in the case of ET it would
> > preclude testing what happens when pyexpat is disabled (remember we were
> > discussing this...). This is because there would be no real way to create
> > new instances of such modules (they would all cache themselves in the
> init
> > function - similarly to what ET now does in trunk, because otherwise some
> > of its global-dependent crazy tests fail).
> >
> > A more radical solution would be to *really* have multiple instances of
> > state per sub-interpreter. Well, they already exist -- it's
> > PyState_FindModule which is the problematic one because it only remembers
> > the last one.
> I'm not sure I understand your diagnosis. modules_per_index (and
> PyState_FindModule) is per-interpreter so we already have a
> per-interpreter state here. Something else must be interferring.
Yes, it's per interpreter, but only one per interpreter is remembered in
state->modules_by_index. What I'm trying to say is that currently two
different instances of PyModuleObject *within the same interpterer* share
the state if they get to it through PyState_FindModule, because they share
the same PyModuleDef, and stat->modules_by_index keeps only one module per

> Note that module state is just a field attached to the module object
> ("void *md_state" in PyModuleObject). It's really the extension modules
> which are per-interpreter, which is a good thing.
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