Call Matlab function from Python script: "not enough values to unpack" ValueError

Question

I called a Matlab function from Python, the function has 3 outputs. Python gives ValueError: not enough values to unpack (expected 3, got 2). Here's the Matlab function testCalledByPython:

function [otpt1, otpt2, otpt3] = testCalledByPython(inpt)
otpt1 = rand(inpt, inpt);
otpt2 = magic(inpt);
otpt3 = zeros(inpt, inpt);

This is the Python script to call the above function:

#!/usr/bin/env python3
import matlab.engine
eng = matlab.engine.start_matlab()
otpt1, otpt2, otpt3 = eng.testCalledByPython(2)

If I replace the last line by otpt1, otpt2 = eng.testCalledByPython(2), it runs, but the outputs are:

otpt1, otpt2
Out[5]: 
(matlab.double([0.8147236863931789,0.12698681629350606]),
 matlab.double([0.9057919370756192,0.9133758561390194]))

Obviously these are wrong outputs. How can I solve it?

one-hand-octopus · Accepted Answer · 2018-10-30 16:29:15Z

2

I found the solution myself. It's actually explained in Matlab Documentation: https://uk.mathworks.com/help/matlab/matlab_external/call-matlab-functions-from-python.html

So knowing there are 3 outputs, the way to do it is changing last line of Python script to:

otpt1, otpt2, otpt3 = eng.testCalledByPython(2, nargout = 3)

answered Oct 30, 2018 at 16:29

one-hand-octopus

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Zubair Adil · Accepted Answer · 2022-11-05 06:47:33Z

0

I also had the similar problem, however I was using the runtime method without API engine.

So knowing that I required/expected x outputs from the function, I added the argument of nargout = x inside the function that I wanted to use or get output from.

answered Nov 5, 2022 at 6:47

Zubair Adil

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