How to pass Exception as an argument?

[gandalf15]

Hello and Thanks for your help in advance.

The problem:
I am trying to pass an exception as an argument to python function. I think that this should work because it works in Python. For example I can do this.

>>> print(ValueError)
<class 'ValueError'>

or more practical thing like this

>>> import asyncio
>>> f = asyncio.Future()
>>> f.set_exception(RuntimeError("UH-OH"))
>>> f
<Future finished exception=RuntimeError('UH-OH')>

Now, if I try to do the same thing in C++ using pybind11 like this

py::print(py::value_error());

I get this error message "Unable to convert call argument '0' of type 'value_error' to Python object". I think that this is expected because as the documentation says here "Note that pybind11 wrappers around Python exceptions such as pybind11::value_error are not Python exceptions; they are C++ exceptions". Therefore, I tried to cast the error like this.

py::print(py::cast(py::value_error()));

then I got this error message "Unable to convert call argument '0' of type 'object' to Python object" which is a bit confusing. The last thing that I tried was to create exception using Python C API. Then I got this error message "Unable to convert call argument '0' of type '_object*' to Python object".

My goal:
I want to register a custom exception like advised in the documentation py::register_exception<CppExp>(module, "PyExp"); and then being able to pass this custom exception as an argument to a python function. Here is a full example.

py::register_exception<CustomCppExc>(module, "CustomPyExc");

auto py_future = py::module_::import("asyncio").attr("Future")();
py_future.attr("set_exception")(CustomCppExc);

I understand that pybind11 does not have support for asyncio. However, this has nothing to do with asyncio module. The underlying problem is that I am unable (I don't know how) to pass exception as an argument to python function.

Thank you!