Why does accessing Python object in Simulink error with "Attempt to extract field 'path' from 'mxArray'."

I'd like to import a Python module in Simulink, and then use the properties/functions of the module. In this example, I want to access the properties and call functions of "out_class_instance".

function out_class_instance = my_matlab_func()
    coder.extrinsic('py.my_python_file.my_python_class')
    out_class_instance = py.my_python_file.my_python_class('exampleInput');
end 

When using either a 'MATLAB Function' or 'MATLAB System' block, I get the following error message:

Simulink detected an error 'Attempt to extract field 'path' from 'mxArray'.'

Answer 1

[MathWorks Support Team]

This error occurs because the Simulink model is using code generation. When using 'coder.extrinsic', the model imports the Python class instance as an 'mxArray', instead of a Python object. This occurs with both MATLAB Function and System blocks.

To access Python objects in Simulink workflows, use a System block. Remove any calls to 'coder.extrinsic' and ensure the system block property 'simulate using' is set to 'interpreted execution'. Take the example Python Code:

class person:
    def __init__(self, name):
        self.name = name
        self.age = 0
    
    def getAge(self):
        return self.age

    def haveBirthday(self):
        self.age += 1

In a separate file, create a MATLAB function to return an instance of this class.

function person_instance = my_matlab_func()
    person_instance = py.my_python_file.person('Jane Doe');
end

Create a custom MATLAB System class component to be used by the System block. Assign the imported Python module to a private property of this MATLAB class. Include needed Propagation Methods in the class definition.

classdef myComponent <  matlab.System
    properties (Access = private)
        MyPerson
    end

    methods (Access = protected)
        function setupImpl(obj)
            obj.MyPerson = my_matlab_func();
        end

        function [y] = stepImpl(obj, x)
            % access property
            y = x; 
            obj.MyPerson.haveBirthday();
            disp(obj.MyPerson.getAge());
        end

        % 'Propagation Methods'
        function out = getOutputSizeImpl(obj)
            % Return size for each output port
            out = [1 1];
        end

        function out = getOutputDataTypeImpl(obj)
            out = 'double';
        end

        function out = isOutputComplexImpl(obj)
            out = false;
        end

        function out = isOutputFixedSizeImpl(obj)
            out = true;
        end

    end
end