Passing varargin to class function from Python with MATLAB.Engine

I have the following MATLAB class function that I need to call from Python:

classdef CF1 < PROBLEM
    methods
        %% Default settings of the problem
        function Setting(obj)
            obj.M = 2;
            if isempty(obj.D); obj.D = 10; end
            obj.lower    = zeros(1,obj.D);
            obj.upper    = ones(1,obj.D);
            obj.encoding = ones(1,obj.D);
        end
        %% Calculate objective values and constraint violations
        function Population = Evaluation(obj,varargin)
            X  = varargin{1};
            X  = max(min(X,repmat(obj.upper,size(X,1),1)),repmat(obj.lower,size(X,1),1));
            D  = size(X,2);
            J1 = 3 : 2 : D;
            J2 = 2 : 2 : D;
            PopObj(:,1) = X(:,1)   + 2*mean((X(:,J1)-repmat(X(:,1),1,length(J1)).^(0.5*(1+3*(repmat(J1,size(X,1),1)-2)/(D-2)))).^2,2);
            PopObj(:,2) = 1-X(:,1) + 2*mean((X(:,J2)-repmat(X(:,1),1,length(J2)).^(0.5*(1+3*(repmat(J2,size(X,1),1)-2)/(D-2)))).^2,2);
            PopCon      = 1 - PopObj(:,1) - PopObj(:,2) + abs(sin(10*pi*(PopObj(:,1)-PopObj(:,2)+1)));
            Population  = SOLUTION(X,PopObj,PopCon,varargin{2:end});
            obj.FE      = obj.FE + length(Population);
        end
        %% Generate points on the Pareto front
        function R = GetOptimum(obj,N)
            R(:,1) = (0:1/20:1)';
            R(:,2) = 1 - R(:,1);
        end
    end
end

I can evaluate the methods 'Setting()' and 'GetOptimum()' easily with the following Python code:

import numpy as np
import matlab.engine
eng = matlab.engine.start_matlab()
% Set the correct path with eng.cd('/path/to/CF1.m', nargout=0)
prob = eng.CF1()
eng.Setting(prob, nargout=0)
out = eng.GetOptimum(prob, 10)
print(np.array(out))

But I can't seem to match the 'varargin' input variable correctly from Python. I have tried passing a matlab.double(10 * [1.0]), a Python list (10 * [1.0]), a Python dictionary ({"1": 10 * [1.0]}), but I get the following errors for the first two and last attempt, respectively:

matlab.engine.MatlabExecutionError: Undefined function 'Evaluation' for input arguments of type 'CF1'
    
ValueError: invalid field for MATLAB struct

Any guidance on how to match the expected varargin argument from python would be much appreciated!

Juan

Answer 1

[Shubham]

Hey Juan,

It seems that you require help when calling MATLAB functions from python using MATLAB Engine API that accept "varargin" as argument to the function call. The "varargin" keyword is used in MATLAB to accept variable number of input arguments.

In order to call the "Evaluation" method, you should pass the arguments directly after the object ("prob"). The first argument expected by "Evaluation" method is a matrix ( X=varargin{1}; ).

Passing a "matlab.double()" should not produce an error and can be passed into the "Evaluation" method.

I have modified the provided code snippets and tried at my end:

classdef CF1 
    methods
        %% Calculate objective values and constraint violations
        function Population = Evaluation(obj,varargin)
            X  = varargin{1};
            Population = X.*2;
        end
        %% Generate points on the Pareto front
        function R = GetOptimum(obj,N)
            R(:,1) = (0:1/20:1)';
            R(:,2) = 1 - R(:,1);
        end
    end
end

Python script:

import numpy as np
import matlab.engine
eng = matlab.engine.start_matlab()
# Set the correct path with eng.cd('/path/to/CF1.m', nargout=0)
prob = eng.CF1()
out = eng.GetOptimum(prob, 10)
print(" printing output of GetOptimum")
print(np.array(out))
X = np.random.rand(2, 2)
print(" printing random matrix of 2x2")
print(X)
print(" printing output of Evaluation method")
X_matlab = matlab.double(X.tolist())
population = eng.Evaluation(prob, X_matlab, nargout=1)
print(population)

Produced output:

Since you are using the input to "Evaluation" method as a matrix, you should keep in mind the resulting MATLAB data types when calling a function. The python dict is converted to a struct. You can have a look at the following documentation:

https://www.mathworks.com/help/matlab/matlab_external/pass-data-to-matlab-from-python.html

For converting a python dict to MATLAB struct, please refer to this MATLAB Answer:

https://www.mathworks.com/matlabcentral/answers/399639-handle-invalid-fieldnames-when-parsing-python-dict-to-matlab-via-struct

For troubleshooting the Undefined function error, please have a look at the following:

https://www.mathworks.com/help/matlab/matlab_external/troubleshoot-matlab-errors-in-python.html

https://www.mathworks.com/matlabcentral/answers/96005-why-do-i-get-the-error-unrecognized-function-or-variable

I hope this helps!

[Shubham]

Hey @Juan Rasines, did you find the above answer helpful? If so, could you please mark the anwer as accepted.