Pass a reference to C++ container to the Python callback

Hi!

I faced this issue while implementing a Python's os.walk-like function for my tree structure. I hoped to mimic the ability of callback to edit passed list of directories in-place and guide further search.

Suppose we have the following in C++:

#include <pybind11/stl_bind.h>

PYBIND11_MAKE_OPAQUE(std::vector<double>);
namespace py = pybind11;
...

py::bind_vector<std::vector<double>>(m, "d_list", py::module_local(false));
// this will work as expected because I can explicitly point that return value is a reference
m.def("test_static_d", []() -> std::vector<double>& {
	static std::vector<double> v{0, 0};
	return v;
}, py::return_value_policy::reference);

// callback will fail to edit passed vector, because a copy is involved somewhere?
m.def("test_callback", [](std::function<void(std::vector<double>&)> f) {
	std::vector<double> v{0, 0};
	f(v);
	for(const auto& l : v) {
		std::cout << l << ' ';
	}
	std::cout << std::endl;
});

And now the Python code:

In [2]: a = example.test_static_d()

In [3]: a.append(1)

In [4]: a
Out[4]: d_list[0, 0, 1]

In [5]: b = example.test_static_d()

In [6]: b
Out[6]: d_list[0, 0, 1]
# so far so good
In [7]: b is a
Out[7]: True

# callback seems to modify a copy of vector
def f(v) :
    v.append(1.)
    print(v)

# C++ function don't get updated vector after callback invoke
In [1]: example.test_callback(f)
d_list[0, 0, 1]
0 0 

Is there a way to specify that callback should take an opaque container by reference (in order to modify it in-place on Python side)?

Start by reproducing the contrast between test_static_d and test_callback using the opaque std::vector binding from pybind11/stl_bind.h. Trace how the std::function callback argument is converted, then verify that mutations made by the Python callback are visible in the C++ vector after the callback returns.