Pybind11: How to create bindings for a function that takes in a struct as an argument?

Question

Please see below C++ code that I am trying to create python bindings for

struct Config {
  int a;
  int b;
};

void myfunction(const Config &config);

Here is what I have so far,

PYBIND11_MODULE(example, m) {
  py::class_<Config>(m, "Config")
    .def_readwrite("a", &Config::a)
    .def_readwrite("b", &Config::b);

  m.def("myfunction", &myfunction);
}

This compiles, but when I attempt to call the python bindings in python,

import example

config = example.Config;
config.a = 1;
config.b = 2;

example.myfunction(config)

I got the below error

TypeError: myfunction(): incompatible function arguments. The following argument types are supported:
    1. (arg0: example.Config) -> None

Invoked with: <class 'example.Config'>

Can someone let me know what is the right way to create python bindings for a function that takes in a struct as an argument?

Typo: `config = example.Config;` -> `config = example.Config()`? — eyllanesc, Sep 17 '21 at 06:06
@eyllanesc pretty sure it's not, if I did what you suggested, `TypeError: example.Config: No constructor defined!` — user3667089, Sep 17 '21 at 06:09
Why don't you declare a default constructor and associate it with py::init? — eyllanesc, Sep 17 '21 at 06:11
@eyllanesc has provided the answer to the issue as posted (unfortunately as a comment). You pass a class instead of an instance (`Invoked with: ` gives a strong hint of this). Issues with declaring a default constructor sound like a separate issue. But providing the default constructor and then getting the _same_ error, well, code to reproduce that would be useful, because it sounds unlikely. — juanchopanza, Sep 17 '21 at 07:19
@eyllanesc you are right, I got it to work with your hint. If you write up an answer I will accept it. Thanks. — user3667089, Sep 17 '21 at 16:58

score 2 · Accepted Answer · answered Sep 18 '21 at 16:09

As hinted by @eyllanesc, the problem will be solved by adding a default constructor:

PYBIND11_MODULE(example, m) {
  py::class_<Config>(m, "Config")
    .def(py::init<>())
    .def_readwrite("a", &Config::a)
    .def_readwrite("b", &Config::b);

  m.def("myfunction", &myfunction);
}

import example

config = example.Config();
config.a = 1;
config.b = 2;

example.myfunction(config)

Pybind11: How to create bindings for a function that takes in a struct as an argument?

1 Answers1