Why are my graphs closing when I use the matlab api?

I'm having an issue where I run this code in my matlab python api and the graph closes right after all the assets load onto the graph and can't seem to figure out why, I'll leave an example I found that does exactly what i was saying. Any help would be very much appreciated.

import matlab.engine
eng = matlab.engine.start_matlab()
eng.eval("T = readtable('patients.dat');",nargout=0)
eng.eval("S = table2struct(T,'ToScalar',true);",nargout=0)
eng.eval("disp(S)",nargout=0)
D = eng.workspace["S"]
smoker = matlab.logical(D["Smoker"])
pressure = D["Diastolic"]
pressure.reshape((1,100))
pressure = pressure[0]
smoker.reshape((1,100))
smoker = smoker[0]
sp = [p for (p,s) in zip(pressure,smoker) if s is True]
nsp = [p for (p,s) in zip(pressure,smoker) if s is False]
print(len(sp))
print(len(nsp))
sp = matlab.double(sp)
nsp = matlab.double(nsp)
print(eng.mean(sp))
print(eng.mean(nsp))
sdx = eng.linspace(1.0,34.0,34)
nsdx = eng.linspace(1.0,34.0,66)
eng.figure(nargout=0)
eng.hold("on",nargout=0)
eng.box("on",nargout=0)
eng.scatter(sdx,sp,10,'blue')
h = eng.scatter(nsdx,nsp,10,'red')
h = eng.xlabel("Patient (Anonymized)")
h = eng.ylabel("Diastolic Blood Pressure (mm Hg)")
h = eng.title("Blood Pressure Readings for All Patients")
h = eng.legend("Smokers","Nonsmokers")
x = matlab.double([0,35])
y = matlab.double([89.9,89.9])
h = eng.line(x,y,"Color","blue")
h = eng.text(21.0,88.5,"89.9 (Smoker avg.)","Color","blue")
y = matlab.double([79.4,79.4])
h = eng.line(x,y,"Color","red")
h = eng.text(5.0,81.0,"79.4 (Nonsmoker avg.)","Color","red")code

Answer 1

[Robert Snoeberger]

My guess is that MATLAB is exiting when your script finishes. Since you are starting a new MATLAB session, the lifetime of MATLAB is tied to the lifetime of the variable 'eng'. You could consider using a shared MATLAB [1].

[1] https://www.mathworks.com/help/matlab/apiref/matlab.engine.connect_matlab.html

[Robert Snoeberger]

You could also add some code at the end of your script to make Python wait for the figure to be closed. One way might be to loop while the figure handle is valid:

while eng.isvalid(h):
    pass

isvalid(h) will return False when the figure is closed.

[Tianya Duan]

Thank you for your answer, help me a lot.

[Frederik]

This trick helped me too.

Just as little addition; if you call this isvalid(h) right after the code above, it will check for the validity of a line object. It would make a little more sense to check for the validity of a figure handle. Therfore... replace eng.figure(nargout=0) by fh = eng.figure(nargout=0) and check with isvalid(fh)...

thanks a lot