How to bind C++ resources into a Python context manager

[nicholasjng]

Say I have a resource like a simple timer on a class, which makes sense to be acquired and released inside a context manager on the Python side:

  nb::class_<State>(m, "State")
      .def("pause_timing", &State::PauseTiming) # <- C++ state APIs
      .def("resume_timing", &State::ResumeTiming) #

How could I expose this on the Python side as a context manager, say like with State().pause(), where State.pause().__enter__() calls State.pause_timing, and State.pause().__exit__() calls resume_timing? Is there a convenience type for C++ resources to context managers in Python?
If there is none and it's feasible to add, consider me signed up for contribution. Thanks!

[wjakob]

No abstraction for this exists, and there aren't plans to add one. This is how I would do it (likely there are some syntax errors, you will need to adapt it).

struct PauseContext { State &state; };

auto state = nb::class_<State>(m, "State")
   .def(...);

nb::class_<PauseContext>(state, "PauseContext")
    .def("__enter__", [](PauseContext &ctx) { ctx.state.PauseTiming(); })
    .def("__exit__", [](PauseContext &ctx, nb::handle, nb::handle, nb::handle) { ctx.state.ResumeTiming(); },
         nb::arg().none(), nb::arg().none(), nb::arg().none());

state.def("pause", [](State &s) { return PauseContext{s}; }, nb::rv_policy::reference_internal);

[nicholasjng]

Thank you! That was easier than I expected.

[nicholasjng]

One more follow-up question if you allow: Is this way (i.e. binding a C++ object) preferred to a lightweight Python class wrapping around the already exposed State class? I read a bit about Python subclassing of C++ objects in the documentation, but I don't have a feeling for whether the ownership problems presented there would happen in this case.

[wjakob]

Both are fine, no issues with sharing are expected.