GeezerVison

PR summary

As mentioned in #32532 and in the nanobind Porting Guide, type casters may not raise exceptions:

Type casters may not raise C++ exceptions. Both from_python() and from_cpp() must be annotated with noexcept.

Since we used exceptions for error messages, I introduced mpl::defer to defer array creation until after a bound function was called. This PR extends mpl::defer to other types using a new mpl::convert function.

mpl::convert has the following signature:

using py::object; // or using nb::object

template <typename T>
void convert(T &dst, const object &src, const char *arg_name)

Each type caster is replaced by a mpl::convert overload:

void convert(agg::trans_affine &dst, const object &src, const char *arg_name)
{
    // If None assume identity transform
    if (src.is_none()) {
        return;
    }

    auto arr = mpl::array<double, mpl::shape<3, 3>>(src, arg_name);
    auto data = arr.data();

    dst.sx = data[0];
    dst.shx = data[1];
    dst.tx = data[2];
    dst.shy = data[3];
    dst.sy = data[4];
    dst.ty = data[5];
}

mpl::defer will will automatically use the appropriate convert() function when value() is called. This allows us to do the following:

static OutputPointArray
Py_transform_points(const mpl::defer<InputPointArray> &points_obj,
                    const mpl::defer<agg::trans_affine> &trans_obj)
{
    auto points = points_obj.value("points");
    auto trans = trans_obj.value("trans");

    auto result = OutputPointArray(points.shape(0));
    transform_points(points, trans, result);

    return result;
}

Additional Changes

• I removed some unneeded template parameters (template <class PathIterator>) and replaced with the actual class (mpl::PathIterator).
• I renamed some template parameters.
• I removed "py_converters.cpp" as the converter is now inline in "py_converters.h".
• I renamed some function arguments to match those in backend_bases.py.

AI Disclosure

• I generally use AI as a web search engine.
• I asked AI for examples on different approaching to implementing mpl::convert and hooking it up to mpl::defer. I used a variation of one of these approaches and wrote the code myself.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• [/] Plotting related features are demonstrated in an example
• [/] New features and API changes have release notes
• [/] Documentation complies with general and docstring guidelines