Modify default value of std::array using pybind11

Question

My goal is to modify array which are declared in C++ struct and assigned with default value.

I have read this, this, but unfortunately I cannot relate it with my problem.

Sample Code

C++

class Math{
struct Data
{
  std::array<float, 5> id_ = {0}; // all value set to ZERO
  std::array<uint32_t, 5> length_ = {0}; // all value set to ZERO
  std::array<bool, 5> status_ = {0}; // all value set to ZERO
  float x_ = 7.5;
};
};

Binded Code

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

namespace py = pybind11;

PYBIND11_MODULE(do_math, m)
{
    py::class_<Math::Data> Data (m, "Data");
    Data.def(py::init<>())
    .def_readwrite("id_", &Math::Data::id_)
    .def_readwrite("length_", &Math::Data::length_)
    .def_readwrite("status_", &Math::Data::status_)
    .def_readwrite("x_", &Math::Data::x_);
}

Now, I would like to modify all std::array member value. I am only showing here id_.
In python file I can access the id_ member variable and it prints [0.0, 0.0, 0.0, 0.0, 0.0] as well as the x_ which output is 7.5

import do_math
struct_obj = do_math.Data()
print(struct_obj.id_)
print(struct_obj.x_)

Now would like to modify the value of id_ but here I am unable to do it.

struct_obj.id_[2] = 2.2 # cannot modify
struct_obj.x_ = 1.5 # it is modified

Still output of struct_obj.id_ is [0.0, 0.0, 0.0, 0.0, 0.0] while struct_obj.x_ is changed to 1.5. How can I modify the id_ array in python?

Approach has taken so far

By following this answer I have tried to implement but failed.

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <pybind11/complex.h>
#include "pybind11/numpy.h"
#include <pybind11/pytypes.h>

namespace py = pybind11;

PYBIND11_MODULE(do_math, m)
{
    py::class_<Math::Data> Data (m, "Data", py::buffer_protocol());
    Data.def(py::init<>())
    .def_property("id_", [](Math::Data &p) -> py::array {
            auto dtype = py::dtype(py::format_descriptor<float>::format());
            auto base = py::array(dtype, {5}, {sizeof(float)});
            return py::array(
                dtype, {5}, {sizeof(float)}, p.id_, base);
        }, [](Math::Data& p) {});
}

Error Message : error: no matching constructor for initialization of 'py::array' return py::array(

unddoch · Accepted Answer · 2022-10-20 14:50:25Z

1

Is the type of struct_obj.id_ a pythonic list? I assume this is the problem. In such a case, a new pythonic list is created as a copy of the C++ array to offer a convenient Pythonic interface. However, this list is not connected anymore to the underlying C++ object because a list can not be a reference to a std::array.

The solution is either to assign id_ directly instead of id_[2]:

l = struct_obj.id
l[2] = 2.2 # cannot modify
struct_obj.id = l

or to export id_ as an opaque object with PYBIND11_MAKE_OPAQUE.

answered Oct 20, 2022 at 14:50

unddoch

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1 Comment

karim Over a year ago

I have tried to use opaque object but failed. Can you kindly show me an approach on the basis of my example?

karim · Accepted Answer · 2022-10-26 06:16:45Z

As several posts I have found that to wrap std::array there is no ready made solution in pybind11 that's why I have followed this answer to use def_property & lambda expression to assign/modify std::array from Python and return the value to python from C++.

Binded code

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <pybind11/complex.h>
#include "pybind11/numpy.h"
#include <pybind11/pytypes.h>

namespace py = pybind11;

PYBIND11_MODULE(do_math, m)
{
    py::class_<Math::Data> Data (m, "Data");
    Data.def(py::init<>())
    .def_property("id", [](Math::Data &self) {return self.id;},
                        [](Math::Data &self, std::array<float,5>buffer) 
                        {self.id = buffer;});
}

Then usage from Python side

import do_math
struct_obj = do_math.Data()
input_id = [1.1,2.2,3.3,7.8,0.5]
struct_obj.id = input_id # Now id can print the desired output

Although the approach has solved my issue but I am not satisfied. Reasons are following:

Each time if I have to update id array, I have to change input_id. Time consuming and maintenance is hard. But it is the demand of the lambda expression which I have used
Implementation of the wrapper in the binded code seems error-prone to me as I have to each time declare the array (for buffer) which has already declared in cpp header/source file. If I can get a way to write the wrapper in such way which can automatically detect the array(here id) and return the updated/assigned/modified value could be best.

Looking for a better and cleaner solution.

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Modify default value of std::array using pybind11

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Although the approach has solved my issue but I am not satisfied. Reasons are following:

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Sample Code

Approach has taken so far

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Although the approach has solved my issue but I am not satisfied. Reasons are following:

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