type caster that turns values into python exceptions?

[ghorn]

We use std::expected for error handling in C++. In python, we never handle the unexpected case - for all unexpected values we just throw exceptions. I tried writing a type caster that did this automatically by throwing an exception in from_cpp, but I observed that it calls std::terminate and does not simply raise this into a python exception as I expected.

This seems to be intentional behavior - from reading the source and from the answer here #272

Should I abandon this approach? Perhaps the better approach would be to do this in the user binding code, not the type caster code for std::expected.

[wjakob]

Indeed, type casters can't raise by design. std::expected as a C++23 feature is also beyond what can be handled in nanobind, which is a C++17 library.

You could either handle this on the Python interface or fork nanobind to handle std::expected specially. However, I would be unlikely to merge such a feature here, in case that is relevant.

[oremanj]

It is possible for from_cpp to raise an exception, but it has to be done in the Python way: call PyErr_SetXyz or PyErr_Format, then return nullptr.

[wjakob]

Right, that's a good point!