Use py::module_local() for Imath bindings

[JeanChristopheMorinPerso]

This PR fixes an issue reported in Slack (see https://academysoftwarefdn.slack.com/archives/CMQ9J4BQC/p1654121088621319).

The issue in question is that importing OpenTImelineIO in Katana would raise an import error like ImportError: generic_type: type "V2d" is already registered!.

The issue comes from the fact that Katana ships Python bindings for Imath made with Pybind11 and OTIO also provides parts of Imath using Pybind11.

The solution to this problem is to make the classes provided by OTIO module-local (see https://pybind11.readthedocs.io/en/stable/advanced/classes.html#module-local-class-bindings).

To quote the documentation:

When creating a binding for a class, pybind11 by default makes that binding “global” across modules. What this means is that a type defined in one module can be returned from any module resulting in the same Python type. For example, this allows the following:

// In the module1.cpp binding code for module1:
py::class_<Pet>(m, "Pet")
   .def(py::init<std::string>())
   .def_readonly("name", &Pet::name);

// In the module2.cpp binding code for module2:
m.def("create_pet", [](std::string name) { return new Pet(name); });

from module1 import Pet
from module2 import create_pet
pet1 = Pet("Kitty")
pet2 = create_pet("Doggy")
pet2.name()
'Doggy'

When writing binding code for a library, this is usually desirable: this allows, for example, splitting up a complex library into multiple Python modules.

In some cases, however, this can cause conflicts. For example, suppose two unrelated modules make use of an external C++ library and each provide custom bindings for one of that library’s classes. This will result in an error when a Python program attempts to import both modules (directly or indirectly) because of conflicting definitions on the external type:

// dogs.cpp

// Binding for external library class:
py::class<pets::Pet>(m, "Pet")
   .def("name", &pets::Pet::name);

// Binding for local extension class:
py::class<Dog, pets::Pet>(m, "Dog")
   .def(py::init<std::string>());

// cats.cpp, in a completely separate project from the above dogs.cpp.

// Binding for external library class:
py::class<pets::Pet>(m, "Pet")
   .def("get_name", &pets::Pet::name);

// Binding for local extending class:
py::class<Cat, pets::Pet>(m, "Cat")
   .def(py::init<std::string>());

import cats
import dogs
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ImportError: generic_type: type "Pet" is already registered!

Using py::module_local() fixes that for us.

[codecov-commenter]

Codecov Report

Merging #1323 (7c2fd78) into main (bc96625) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1323   +/-   ##
=======================================
  Coverage   86.17%   86.17%           
=======================================
  Files         195      195           
  Lines       19806    19806           
  Branches     2313     2313           
=======================================
  Hits        17068    17068           
  Misses       2174     2174           
  Partials      564      564           

Flag	Coverage Δ
py-unittests	86.17% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ntimelineio/opentimelineio-bindings/otio_imath.cpp	61.53% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bc96625...7c2fd78. Read the comment docs.

[meshula]

Thanks for discovering this solution!