Pybind: Is it possible to inform Python about the names of the arguments for constructors?

Question

In Pybind it is possible to inform Python about the names of the arguments for functions:

m.def("add", &add, "A function which adds two numbers",
  py::arg("i"), py::arg("j"));

(http://pybind11.readthedocs.io/en/stable/basics.html#keyword-arguments)

Is there something similar for constructors? Spyder (Anaconda) already shows the input arguments of functions by default, but for constructors the "help" only shows: (*args, **kwargs).

Answer 1

Yes, in exactly the same way as it is for member functions or functions :)

struct Foo {
    Foo(int x, int y) {}
    void bar(int a, int b) {}
};

PYBIND11_MODULE(cpp_module, m) {
    py::class_<Foo>(m, "Foo")
        .def(py::init<int, int>(), py::arg("x"), py::arg("y"))
        .def("bar", &Foo::bar, py::arg("a"), py::arg("b"));
}

As far as I can see, there is no real difference between using py::arg for functions, member functions or constructors, it works in the same way (including default values etc).

---

Overloading a function (including the constructor) is an interesting case. Since Python has no similar overloading mechanism, this is handled by pybind11. help() will still work, but it'll show something like this:

__init__(...)
    __init__(*args, **kwargs)
    Overloaded function.

    1. __init__(self: some_cpp.Foo, x: int, y: int) -> None

    2. __init__(self: some_cpp.Foo) -> None

As you can see, __init__ itself takes (*args, **kwargs) and this will be what is autocompleted by most IDE's. A way around this would be to use static methods as constructors, this way you can give each constructors a unique name so Python knows about this. For example, Foo Foo::from_ints(int x, int y) and Foo Foo::from_string(std::string s).