Intersection of overloads and std::function<>

[jcelerier]

I'd like to have multiple overloads that take different kinds of functions in parameters:

  .def(
      "add_callback",
      [](foo& addr, std::function<void(int)> clbk) { ... })
  .def(
     "add_callback",
      (foo& addr, std::function<void(foo&, int)> clbk) { ... })

But the following python code fails:

def fun_callback(foo, val):
  print(...)

my_foo.add_callback(fun_callback)

with:

TypeError: fun_callback() missing 1 required positional argument: 'val'

The first overload works, and renaming the second overload to another function name (for instance add_callback2) works too.

Is there a way around this ?

[henryiii]

Quick note: you must be compiling in C++14, or at least with a C++14 supporting compiler, in order for the compiler to be able to tell the difference between differing signatures in std::function. This might be related. (I think GCC 4.9 was the first to support it, for example, but PyBind11 supports GCC 4.8, so it might stem from that)

[YannickJadoul]

Wouldn't the problem be that a Python function cannot 'match a typed overload', in general, because of Python's duck-typing?

[jcelerier]

Quick note: you must be compiling in C++14, or at least with a C++14 supporting compiler,

yep, I am (using g++ 7.2).

Wouldn't the problem be that a Python function cannot 'match a typed overload', in general, because of Python's duck-typing?

hmm... yes, that would make sense. If I understand correctly, there is a single "function" type from Python's side ?

[YannickJadoul]

hmm... yes, that would make sense. If I understand correctly, there is a single "function" type from Python's side ?

I think so. Or not even, actually. There's just 'things that can be called' (i.e., I'm guessing you could perfectly pass an object with the __call__ method implemented). Either way, these things typically only get caught once they are used, in Python.

On the other hand, doesn't Python allow some kind of introspection, either? But then I don't know if pybind uses that.

[YannickJadoul]

On the other hand, doesn't Python allow some kind of introspection, either? But then I don't know if pybind uses that.

Answer: nope, looking at functional.h.

        if (!isinstance<function>(src))
            return false;

// ...

        value = [func](Args... args) -> Return {
            gil_scoped_acquire acq;
            object retval(func(std::forward<Args>(args)...));
            /* Visual studio 2015 parser issue: need parentheses around this expression */
            return (retval.template cast<Return>());
        };

[jcelerier]

okay, I guess I'll just give my function another name ! thanks