Exposing Eigen::Tensor To Python

[afpgit]

I am trying to expose an Eigen tensor to python using pybind11. I can compile everything with no issue and can successfully import it to python. However, the data cannot be converted to a python type. I tried two methods. One is directly exposing the data and second using mapping. Both fail in python environment.

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <pybind11/stl_bind.h>
#include <pybind11/numpy.h>
#include <pybind11/eigen.h>
#include <unsupported/Eigen/CXX11/Tensor>

namespace py = pybind11;

class myclass{
    
    myclass(){
        m_data = new float[m_dim1*m_dim2*m_dim3]; // Contiguous data that represents a three dimensional array
        for(int i = 0; i<m_dim1*m_dim2*m_dim3; i++)
            m_data[i] = i;
        
        m_tensor = Eigen::TensorMap<Eigen::Tensor<float, 3>>(m_data, m_dim1, m_dim2, m_dim3);
    }
    
    Eigen::TensorMap<Eigen::Tensor<float, 3>>& getDataUsingMapping() { Eigen::TensorMap<Eigen::Tensor<float, 3>> temp(m_data, m_dim1, m_dim2, m_dim3);  return temp; }
    Eigen::Tensor<float, 3>& getDataWithoutUsingMapping() { return m_tensor};
    
    
private:
    Eigen::Tensor<float, 3> m_tensor;
    // In fact, m_data, m_dim1, m_dim2, m_dim3 all are
    // read from a data file but for this example let's 
    // assume some values.
    float* m_data; 
    int m_dim1 = 2, m_dim2 = 5, m_dim3 = 10;
}


PYBIND11_MODULE(example, m) {
    py::class_<myclass>(m, "myclass")
        .def(py::init<>())
        .def("getDataUsingMapping", &myClass::getDataUsingMapping, py::return_value_policy::reference)
        .def("getDataWithoutUsingMapping", &myClass::getDataWithoutUsingMapping, py::return_value_policy::reference);
}

I would like to be able to process this 3D array in python with its dimensional information (m_dim1, m_dim2, m_dim3).

Here are the error messages I get after trying to get the data in python.

>>> import example
>>> d = example()
>>>
>>> DataInPython = d.getDataUsingMapping()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Unable to convert function return value to a Python type! The signature was
		(self: example) -> Eigen::TensorMap<Eigen::Tensor<float,3,0,__int64>,0,Eigen::MakePointer>
>>>
>>>
>>> DataInPython = d.getDataWithoutUsingMapping()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Unable to convert function return value to a Python type! The signature was
		(self: example) -> Eigen::Tensor<std::complex<float>,3,0,__int64>

Did you forget to `#include <pybind11/stl.h>`? Or <pybind11/complex.h>,
<pybind11/functional.h>, <pybind11/chrono.h>, etc. Some automatic
conversions are optional and require extra headers to be included
when compiling your pybind11 module.

I tried including all pybdin11 include files but did not fix the problem. Could someone kindly help me?

[ghost]

@afalahat I have the same problem.
Did you fix your code?

[afpgit]

I don't think exposing the tensor is doable as Eigen::Tensor is unsupported.

[bstaletic]

There's always an option of writing a custom caster. That said, a pull request would be welcome. We do need help maintaining the eigen.h header.

[sergiud]

I wrote type casters for Eigen::Tensor of arbitrary rank (and OpenCV's cv::Mat), albeit using C++17 features. Maybe someone finds it useful and possibly can use the implementation as basis for a pybind11 submission.