title: Matplotlib and :RuntimeError: main thread is not in main loop:
body: <p>I'm trying to run multiple repetitive tasks in parallel under python. I'm pretty new to multiprocessing but as all the tasks are independant, I used the simple following piece of code :</p>
  <pre><code>    import numpy as np
      import sys
      import os
      import glob
      import matplotlib.pyplot as plt
      import concurrent.futures as Cfut
      
      def analize(simul, N_thread):
          path = os.getcwd()
  
          print('Analizing topo ...')
          Data = output of some calculations
      
          print('Writing Data ...')
          np.save('Data', Data)
      
          print('Data saved')
          print('Printing figures')
  
          plt.figure()
          plt.plot(Data[0])
          plt.savefig('figure1.pdf')
  
          plt.clf()
          plt.plot(Data[0])
          plt.savefig('figure1.pdf')
  
          plt.close('all')
          print('figures saved')
          os.chdir(path
  
  if __name__ == '__main__':
      list_simul = sorted(glob.glob('Layer*'))
      executor = Cfut.ProcessPoolExecutor(5)
     #executor = Cfut.ThreadPoolExecutor(N_jobs)
      futures = [executor.submit(analize, item, 10) for item in list_simul]
      Cfut.wait(futures)
  </code></pre>
  <p>It ran very well during 3 months, and since since morning I get sometimes the following error :</p>
  <pre><code>Exception ignored in: &lt;bound method Image.__del__ of &lt;tkinter.PhotoImage object at 0x7fac6c187f98&gt;&gt;
      Traceback (most recent call last):
        File &quot;/usr/lib/python3.5/tkinter/__init__.py&quot;, line 3359, in __del__
          self.tk.call('image', 'delete', self.name)
      RuntimeError: main thread is not in main loop
  </code></pre>
  <p>What is odd is that some jobs are done without any problem, and it happens not all the time. With a bit of research, I found a lot of posts with this problem, all of them were related to GUI. I understood the problem, but I guess I should be able to find a workaround since I'm not showing any figure, just creating the object and saving it, and since all the tasks are independent.</p>
  <p>Any hint ?</p>