Python : When trying to plot a second figure, an empty tkinter window is displayed

[A Former User]

Hello,

I have another problem. When trying to integrate my code as a pluging in QGIS, I get this error :

Exception in thread Thread-1:
Traceback (most recent call last):
File "C:\OSGEO4~1\apps\Python27\lib\threading.py", line 808, in __bootstrap_inner
self.run()
File "C:\OSGEO4~1\apps\Python27\lib\threading.py", line 761, in run
self.__target(*self.__args, **self.__kwargs)
File "C:\OSGEO4~1\apps\Python27\lib\site-packages\matplotlib\pyplot.py", line 253, in show
return _show(*args, **kw)
File "C:\OSGEO4~1\apps\Python27\lib\site-packages\matplotlib\backend_bases.py", line 193, in call
self.mainloop()
File "C:\OSGEO4~1\apps\Python27\lib\site-packages\matplotlib\backends\backend_qt5.py", line 150, in mainloop
signal.signal(signal.SIGINT, signal.SIG_DFL)
ValueError: signal only works in main thread

[jsulm]

@EJWA said in Python : When trying to plot a second figure, an empty tkinter window is displayed:

ValueError: signal only works in main thread

This error message actually tells you what is wrong: signal() can only be used in main thread. You can find this information in Python documentation: https://docs.python.org/2/library/signal.html
"Some care must be taken if both signals and threads are used in the same program. The fundamental thing to remember in using signals and threads simultaneously is: always perform signal() operations in the main thread of execution."

[A Former User]

Hello @jsulm and thanks for your reply.

I have never used signals and I have no idea how to use it in my case. Could you give me some examples ?

Thanks in advance.

[jsulm]

@EJWA To clarify: it's not about Qt signals, its about UNIX signals. I don't know your code, so I don't know where this signal() Python stuff is used. What exactly are you integrating and how? And when does this error occur?
Are you using any threads?
"File "C:\OSGEO4~1\apps\Python27\lib\site-packages\matplotlib\backends\backend_qt5.py", line 150, in mainloop
signal.signal(signal.SIGINT, signal.SIG_DFL)"
C:\OSGEO4~1\apps\Python27\lib\site-packages\matplotlib\backends\backend_qt5.py - is this what you're integrating? Its the file where the error occurs.

[A Former User]

Hello,

I am integrating my code as a plugin in QGIS using this method http://www.qgistutorials.com/fr/docs/building_a_python_plugin.html.

It consists on GUI interface which have a button when clicked generate graphs with matplotlib. Every time, we select an iteration in GUI, a plot will be drawn on the graph. So, the graph can have many plots.

Before, I have a problem with graph windows : I can't control it : I can't close it or save or zoom, so I use threading which helps me. Here is what I put in my code (function used when the button "generate graph" is clicked) :

fig = plt.figure(self.numFiguresPlot2_1) 
 fig.canvas.set_window_title(self.DropDownListDisplaySector2_1.currentText())
 
 x = np.arange(0, self.stockParam.nTotZones, 1)
 y = pd.to_numeric(matrix4["Price"])
 print("Moyenne = ")
 print(y.mean(0))
 z = pd.to_numeric(matrix4["Adjust"]*y)/100 + y
  # plot data
 if(firstPlot):
         price = plt.plot(x, y, label = ("Price"))
         shadowPrice = plt.plot(x, z, label = ("Price + adjust for iteration "+self.DropDownListDisplayIter2_1.currentText()))
 
plt.ion()
plt.legend() 
draw_thread = threading.Thread(target=plt.show)  
draw_thread.start()
plt.ioff()

[jsulm]

@EJWA In Qt you should never do any UI related stuff in other thread than GUI thread! This is not supported.

[A Former User]

@jsulm So any alternatives to do that ? I have used other solutions like subprocess http://stackoverflow.com/questions/36675269/cannot-move-matplotlib-plot-window-and-exit-it-using-red-x-button and I don't get the result what I want. I am really confused to do that.

[jsulm]

@EJWA "Before, I have a problem with graph windows : I can't control it : I can't close it or save or zoom" - you should investigate why. It should work without threads.

[A Former User]

@jsulm When I investigate I found this answer ( http://stackoverflow.com/questions/36675269/cannot-move-matplotlib-plot-window-and-exit-it-using-red-x-button)

"his is because your matplotlib figure and your PyQt GUI are both running in the same main thread. Since they are in the main thread, only one of them has the CPU for itself"

If it is not correct, I must search for another reason.

[jsulm]

@EJWA In the link you posted there is a workaround:

import subprocess
...

    # When you click the button in your PyQT GUI,
    # Create a new process:
    myMatProcess = subprocess.Popen(['python', 'myGraph.py'], shell=True)

[A Former User]

Hello,

Finally, I found the solution .

First, in my script I put :

import matplotlib
matplotlib.use('Qt4Agg')

then in function of plot I put :

plt.legend()
plt.show()
plt.draw()

and it works for me correctly except there is a warning :

QCoreApplication::exec: The event loop is already running

[A Former User]

import matplotlib
matplotlib.use('Qt4Agg')

plt.legend()
plt.draw()
plt.show(block=False)