Don't know how to use py::register_exception. What goes in the place of module?

[inhahe]

According to the instructions, register_exception is used like this: py::register_exception<CppExp>(module, "PyExp");

I'm not sure what's supposed to go in module's place. I couldn't find any actual functional examples online.

module is also used as a placeholder in the instructions for PYBIND11_MODULE(). In that function, I just used an arbitrary name (PyMusic) that I decided my module should be called, without quotes. That part gives me no compilation errors. But using the same name in place of module in py::register_exception<CppExp>(PyMusic, "PyExp");, I get an undeclared identifier error. I tried it with quotes around 'PyMusic', too, and got a different error in that case.

Here's my code:

#include <pybind11/pybind11.h>
#include <JuceHeader.h>

namespace py = pybind11;
using namespace std;
using namespace juce;

class ErrorLoadingPlugin : public std::exception 
{
public:
  char* what() 
  {
    return "Error: Could not load plugin.";
  }
};

class Plugin
{
  juce::AudioPluginFormatManager manager;
  std::unique_ptr< AudioPluginInstance > instance;
public:
  Plugin(string filepath, string formatname, int bitrate, int buffersize)
  {
    PluginDescription desc;
    desc.fileOrIdentifier = filepath;
    desc.pluginFormatName = formatname;
    juce::String errmsg;
    this->instance = manager.createPluginInstance(desc, bitrate, buffersize, errmsg);
    if (this->instance == nullptr)
    {
      throw ErrorLoadingPlugin();
    };
  };
};

PYBIND11_MODULE(PyMusic, m) 
{
  py::register_exception<ErrorLoadingPlugin>(PyMusic, "ErrorLoadingPlugin");
  py::class_<Plugin>(m, "Pet")
    .def(py::init<string, string, int, int>());
}

Second question: Not knowing what to put in place of module aside, is this how I should be using it? I just want to throw a custom exception. I couldn't find a way to just directly throw a custom exception, so I'm using register_exception and throwing a C++ exception and hoping it catches it and throws it in Python.

Thanks.

[inhahe]

I then tried py::register_exception<ErrorLoadingPlugin>(m, "ErrorLoadingPlugin"); as a shot in the dark, then saw later that that's how it's done in a code snippet in the docs:

PYBIND11_CONSTINIT static py::gil_safe_call_once_and_store<py::object> exc_storage;
exc_storage.call_once_and_store_result(
    [&]() { return py::exception<MyCustomException>(m, "MyCustomError"); });
py::register_exception_translator([](std::exception_ptr p) {
    try {
        if (p) std::rethrow_exception(p);
    } catch (const MyCustomException &e) {
        py::set_error(exc_storage.get_stored(), e.what());
    } catch (const OtherException &e) {
        py::set_error(PyExc_RuntimeError, e.what());
    }
});

But when I tried it, I got 'char *ErrorLoadingPlugin::what(void)': cannot convert 'this' pointer from 'const CppException' to 'ErrorLoadingPlugin &'

[inhahe]

I read some more of the docs, that I must have missed the first time, and realized that registering an exception is more complex than what I was doing. The docs don't explain the necessity for every line in the code snippet, such as PYBIND11_CONSTINIT static py::gil_safe_call_once_and_store<py::object> exc_storage;, but nonetheless I copied it almost verbatim into my program.

Here's the relevant part of my code now:

PYBIND11_MODULE(PyMusic, m) 
{
  py::class_<Plugin>(m, "Plugin ")
    .def(py::init<string, string, int, int>());
  PYBIND11_CONSTINIT static py::gil_safe_call_once_and_store<py::object> exc_storage;
  exc_storage.call_once_and_store_result(
    [&]() { return py::exception<ErrorLoadingPlugin>(m, "ErrorLoadingPlugin"); });
  py::register_exception_translator([](std::exception_ptr p) {
    try {
      if (p) std::rethrow_exception(p);
    }
    catch (const ErrorLoadingPlugin& e) {
      py::set_error(exc_storage.get_stored(), e.what());
    }
  });
}

And here's the compile-time error I get: 'char *ErrorLoadingPlugin::what(void)': cannot convert 'this' pointer from 'const ErrorLoadingPlugin' to 'ErrorLoadingPlugin &' at this line: py::set_error(exc_storage.get_stored(), e.what());

This in Visual Studio 2022, btw.

[inhahe]

So, since I can't figure out how to throw a custom exception, I'm just doing throw new py::import_error();, but I don't know how to set any exception text, so I have to just print the error message beforehand using py::print, which is awkward.

Also, for some reason when I throw that exception, it results in an "unknown exception" or whatever in Python, which is odd, first because it's an exception defined within pybind11, and second because in the docs it's listed as one of the exception that Pybind11 automatically converts to Python exceptions, so I'm not sure if I'm doing something wrong here.