Interop between 2 nanobind modules

[jjcasmar]

I have a library A which is designed to be extensible via templates. This library A also expose some of its core functionality to python using nanobind.

Now I am writing a library B which is based on A, and it uses some of the core functionalities in A. I am also exposing B to python using nanobind.

Is it possible to pass objects from libA module to libB module?

[wjakob]

Yes.

[jjcasmar]

Do I need to do anything special?
Or can B::foo receive an A::object?

in module A

nb.class_<A::obj>(ma, "obj");

in module B

mb.def("foo", [](A::obj ao) {};);

import mb
import ma
mb.foo(ma.obj)

[wjakob]

This should just work fine. Such a case is even included in the test suite.

[jjcasmar]

Just a final question, if libA exposes an type to python, can I return that same type from libB?

in module A

nb.class_<A::obj>(ma, "obj");

in module B

mb.def("foo", []() {return A::obj{}};);

I tried to do something like that, but nanobind complains in python side that A::obj its not exposed to python (from B). Is there any way to achieve this?

[oremanj]

That should work fine as long as the nb::class_ statement defining the binding for A::obj has actually executed by the time you try to return an instance of A::obj to Python. You need to make sure the bindings for A have been imported on the Python side; it's not sufficient to just link libA.so into the extension module for B. A call like nb::module_::import_("module_a"); inside the NB_MODULE for module B might be helpful.

[dograt]

I just asked a similar question. What are the limitations of interop between nanobind libraries in relation to ABI compatibility? Specifically, nanobind versions?

I ask because things "just work" when compiling both libraries together using an editable install. However, when I try to pull one in as a wheel, I get an error like this:

=================================== FAILURES ===================================
_____________________________ test_type_match _____________________________
    def test_type_match():
        """Test that bindings correctly import interop types."""
>       thing = make_default_thing()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: Unable to convert function return value to a Python type! The signature was
E           make_default_thing() -> lib_a::Thing
projects/lib_b/tests/interop/test_liba_types.py:11: TypeError
=========================== short test summary info ============================
FAILED projects/lib_b/tests/interop/test_types.py::test_type_match - TypeError: Unable to convert function return value to a Python type! The signature was
    make_default_thing() -> lib_a::Thing

I've just tracked down that this is because the lib_A wheel is built with a newer version of nanobind (2.9.2 vs 2.7 for lib_B)