Wrapping empty list works, wrapping empty set results in a TypeError

[francesco-ballarin]

Issue description

I am trying to wrap a C++ function which takes as input a set with pybind11. If the set is non-empty, wrapping works fine,
while if the set is empty I get errors such as

TypeError: print_set(): incompatible function arguments. The following argument types are supported:
    1. (arg0: Set[int]) -> None

I realize that the type int cannot be deduced from an empty set. However, wrapping of empty lists succeeds.
Is it possible to extend set conversion to the case of empty set?

Thanks.

Reproducible example code

import cppimport

def compile_module():
    cpp_code = """
    <%
    setup_pybind11(cfg)
    %>

    #include <pybind11/pybind11.h>
    #include <pybind11/stl.h>
    #include <iostream>

    void print_list(const std::vector<int> & list){
        std::cout << "List: ";
        for (auto& i: list) {
            std::cout << i << " ";
        }
        std::cout << std::endl;
    }

    void print_set(const std::set<int> & set){
        std::cout << "Set: ";
        for (auto& i: set) {
            std::cout << i << " ";
        }
        std::cout << std::endl;
    }

    void print_list_of_lists(const std::vector<std::vector<int>> & lists){
        std::cout << "Lists: ";
        for (auto & list: lists) {
            for (auto& i: list) {
                std::cout << i << " ";
            }
            std::cout << ", ";
        }
        std::cout << std::endl;
    }

    void print_list_of_sets(const std::vector<std::set<int>> & sets){
        std::cout << "Sets: ";
        for (auto & set: sets) {
            for (auto& i: set) {
                std::cout << i << " ";
            }
            std::cout << ", ";
        }
        std::cout << std::endl;
    }

    PYBIND11_MODULE(pybind_tester, m)
    {
      m.def("print_list", &print_list);
      m.def("print_set", &print_set);
      m.def("print_list_of_lists", &print_list_of_lists);
      m.def("print_list_of_sets", &print_list_of_sets);
    }
    """

    open("pybind_tester.cpp", "w").write(cpp_code)

    return cppimport.imp("pybind_tester")

module = compile_module()

# Print non-empty containers
module.print_list([8, 9])
module.print_set({1, 2})

# Print empty containers
module.print_list([])
try:
    module.print_set({})
except TypeError:
    print("Set: failure")
else:
    raise RuntimeError("Expecting failure")

# Print list of containers with all non-empty elements
module.print_list_of_lists([[8, 9], [1, 2, 3], [4]])
module.print_list_of_sets([{1, 2}, {6}, {4, 3}])

# Print list of containers with all non-empty elements
module.print_list_of_lists([[8, 9], [1, 2, 3], [4], []])
try:
    module.print_list_of_sets([{1, 2}, {6}, {4, 3}, {}])
except TypeError:
    print("Sets: failure")
else:
    raise RuntimeError("Expecting failure")

[bstaletic]

This is interesting. For some reason py::isinstance<py::set>(src) evaluates to False in this case...

[likesum]

Bump. Is there any update on fixing this bug?