"Too Many Output Arguments" Error Calling imwrite from Python engine

I have a MATLAB function get_mask defined like so:

 function M = get_mask(path)
    I = imread(path);
    M = im2bw(I, graythresh(I));
 end

The image I'm passing in is a 16bpp grayscale TIFF LZW compressed image. I call it from Python using the Python engine:

 eng = matlab.engine.start_matlab()
 result = eng.get_mask('some_path');

I then pass the result to imwrite:

 eng.imwrite(result, r'ml_result.tif', nargout=0)
 eng.quit()

The call to imwrite takes ages (3-4 minutes, a separate issue...) before failing with a "Too Many Output Arguments" error. I double checked the type of 'result' and it is indeed a 1040x1392 logical as I expected. However, if I do this instead:

 eng.imwrite(matlab.logical([[0, 1, 0], [1, 0, 1]]), r'C:\ml_result.tif', nargout=0)

It works, no problem. I don't understand what the difference is here.

Answer 1

[E Swan]

And, I just figured it out on my own. I lied a bit in my example (shame on me) as I was actually creating one engine instance to call get_mask, stopping it, and then creating another for the call to imwrite. Apparently that is a no-no and, for whatever reason, resulted in this error.

It still takes a while to run, but that's a separate topic.

Answer 2

[Bo Li]

By default, the number of output arguments is 1 in Python Engine. nargout needs to specified if more or less results are expected:

http://www.mathworks.com/help/matlab/apiref/matlab.engine.matlabengine-class.html

[E Swan]

Thanks. As you can see in the example, I did set nargout to 0. The problem was using data obtained from one instance of the engine and passing it to another after quitting the first.