Python call matlab function 'plot', The figure disappeared in few seconds.

When I use python call the matlab funtion plot, the figure only exists few seconds, then it colsed automatically. Why does have this phenomenon occur? How could I change or add something? Thank you very much.

AddTset.m

function c = AddTest(a,b)
   c = a + b;
   figure(1)
   plot(a,c,'DisplayName','test');
   %pause(30);
end

pyCallMat.py

import matlab.engine

if __name__ == '__main__':
    eng = matlab.engine.start_matlab()
    a = eng.AddTest(1.0,2.0) 
    print(a)
    b = eng.sqrt(4.) 
    eng.figure(3.0)
    eng.plot(a, b, nargout=0)

    print(b)

[Mohammad Sami]

I presume this maybe because the variable eng get cleared after your function is finished executing. this might perhaps close the matlab engine.

[Tianya Duan]

Hi，Sami, firstly thank you quick answer. I also think the eng may be the root cause, but do you know how to solve it?

[Tianya Duan]

The question has been solved by Chinese matlab support team.

Two methods:

1. add a line of the following command

input("Press Enter to continue...")

2. use the following answer of the website

isvalid function is more useful if you want to write a GUI.

while eng.isvalid(h):
    pass

[Villa Xue]

Thank you a lot, It works when use: input("Press Enter to continue...") in python

Answer 1

[Hornett]

The issue you're encountering is likely due to the fact that the MATLAB engine for Python does not keep the MATLAB session open once the Python script finishes executing. As a result, any figures created in MATLAB are closed when the session ends.

To keep the MATLAB figure open, you can use the matlab.engine's blocking parameter to keep the MATLAB session alive. Another approach is to add a pause in your MATLAB function to keep the figure open for a specified amount of time.

Solution 1: Using pause in MATLAB Function

You can add a pause statement in your MATLAB function to keep the figure open for a specified duration:

matlab

function c = AddTest(a, b)
c = a + b;
figure(1)
plot(a, c, 'DisplayName', 'test');
pause(10);  % Pause for 10 seconds
end

Solution 2: Keeping MATLAB Session Alive

Modify your Python script to keep the MATLAB session alive using the blocking parameter:

python

import matlab.engine
if __name__ == '__main__':
    eng = matlab.engine.start_matlab()
    a = eng.AddTest(1.0, 2.0, nargout=1)
    print(a)
    b = eng.sqrt(4.0)
    # Keep the MATLAB session alive
    eng.set_param('0', 'SimulationCommand', 'start', nargout=0)
    input("Press Enter to close MATLAB session...")
    eng.quit()

Solution 3: Using matplotlib for Plotting in Python

If you don't need to plot in MATLAB specifically, you can use Python's matplotlib to plot the results. This way, you keep the plotting within the Python environment:

python

import matlab.engine
import matplotlib.pyplot as plt

if __name__ == '__main__':
    eng = matlab.engine.start_matlab()
    a = eng.AddTest(1.0, 2.0, nargout=1)
    print(a)
    b = eng.sqrt(4.0)
    # Plot using matplotlib
    plt.figure()
    plt.plot([1.0], [a], label='test')
    plt.xlabel('a')
    plt.ylabel('c = a + b')
    plt.title('Plot of a vs c')
    plt.legend()
    plt.grid(True)
    plt.show()
    print(b)

Summary

• Solution 1: Add a pause statement in your MATLAB function to keep the figure open for a specified duration.
• Solution 2: Modify your Python script to keep the MATLAB session alive using the blocking parameter.
• Solution 3: Use Python's matplotlib for plotting instead of MATLAB.

Choose the solution that best fits your needs. If you still want to use MATLAB for plotting, Solution 1 or Solution 2 would be appropriate. If you are open to using Python for plotting, Solution 3 is a good option.