pybind11 basic callback, incompatible function signature error

Question

I cannot for the life of me get a very basic Python callback function to work in an extension module built with pybind11. I am trying to follow the example here , but I guess I must be misunderstanding something.

The C++ code is as follows:

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

namespace py = pybind11;

void run_test(const std::function<int(int)>& f)
{
   // Call python function?
   int r = f(5);
   std::cout << "result: " << r << std::endl;
}

PYBIND11_PLUGIN(mymodule)
{
    py::module m("mymodule");
    m.def("run_test", &run_test);
    return m.ptr();
} 

And the Python code which uses this module is

import mymodule as mm

# Test function
def test(x):
  return 2*x

mm.run_test(test)

However I get this error:

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    mm.run_test(test)
TypeError: run_test(): incompatible function arguments. The following argument types are supported:
    1. (arg0: std::function<int (int)>) -> None

Invoked with: <function test at 0x2b506b282c80>

Why doesn't it think the function signatures match? I tried to match the examples but I guess I must misunderstand something...

Answer 1

Ahh ok, I misunderstood why std::function was used in that example. That was for a more complicated double-callback, where a C++ function was passed to Python and then back into C++, to check that it survived the journey. For just calling a Python function one needs to use py::object and cast the result to a C++ type (as described here ):

void run_test(const py::object& f)
{
   // Call python function?
   py::object pyr = f(5);
   int r = pyr.cast<int>();
   std::cout << "result: " << r << std::endl;
}

Answer 2

我相信您的问题是您忘记了#include <pybind11/functional.h> 。