[Import-SIG] On singleton modules, heap types, and subinterpreters

[Petr Viktorin]

On Tue, Jul 28, 2015 at 3:52 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Sun, 26 Jul 2015 12:39:21 +0200
> Petr Viktorin <encukou at gmail.com> wrote:
>>
>> So, what options are there for methods of extension classes to get a
>> hold of the module object (or module state)?
>
> The "obvious" answer is to use the C equivalent of
> `sys.modules[__name__]`. It should always do the right thing. However,
> it's also quite inefficient and cumbersome to write.

We don't have access to globals, so we can't use __name__.
We can't use a hardcoded module name either: the fully-qualified name
depends on the loading mechanism and/or location of the shared library
in the filesystem. (Extension modules usually only hard-code the last
part of the module name, without the package prefix.)

Another problem is that "sys.modules" can be modified by the user. For
Python programs, where you just get a LookupError when something isn't
found, looking things up in sys.modules is OK. But here we're
retrieving C-level state: when the wrong module isn't found (or we
find nothing at all), a segfault is one of the better things that can
happen.