RuntimeError: main thread is not in main loop with Matplotlib and Flask

Question

I'm using flask, matplotlib to save image and tensorflow to create a session. I'm getting the above error when I run the following code. Does the flask route run on a separate thread? How can I make fig.saveFig piece of code run on the Main thread. Thanks a lot

 @app.route('/open', methods = ['GET', 'POST'])
 def sendOutput():
     global loss,a2,xP,yP,scale,sess,fig
     test_X,test_Y = own_model.getEvaluateData(scale)
     cost,ans = sess.run([loss,a2],feed_dict={xP:test_X,yP:test_Y})
     d = np.array(ans) - np.array(test_Y)
     val = hist(d,100)
     sess.close()
     fig.saveFig('abc.png') //Errror on this line

score 84 · Answer 1 · answered Jul 04 '18 at 17:23

84

I was on the same situation, Flask with Matplotlib combo. What worked for me is to specify Agg as Matplotlib backend.

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Your code here

You can refer to Matplotlib documentation (Matplotlib in a web application server) for the details.

answered Jul 04 '18 at 17:23

crispy

959
8
9

13

Note you need to put this code *before* any other imports, which might be already importing matplotlib or setting the Tkinter backend. – ZeitPolizei Jul 05 '18 at 17:08
9

This works like a charm, Btw what does `Agg` actually do? – Sundeep Pidugu Nov 22 '19 at 09:20
2

Hey, thanks for the answer. My code works now but I still get a Warning UserWarning: Starting a Matplotlib GUI outside of the main thread will likely fail. Do you know about anything I could do to not get this warning? – Riptide Nov 02 '20 at 10:07

score 3 · Answer 2 · answered Apr 29 '18 at 14:26

apparently this is an issue with TKinter. You don't have your full error message posted (you really should do that), but I'm working on something very similar so I'm 99.9% sure your problem is the same as mine. I'm building an app with tensorflow, flask, blah blah. I get the same error, intermittently (sometimes it works). Here are some links to similar questions. I hope this helps! I think my solution will be moving away from matplotlib.

score 0 · Answer 3 · answered Apr 06 '23 at 11:04

Threading issues. I had the same issue when working with lime. Here is what fixed it for me.

I changed from:

if __name__=="__main__":
    app.run(host="0.0.0.0", port=5000)

To:

if __name__ == '__main__':
    from werkzeug.serving import run_simple
    run_simple('localhost', 5000, app)

RuntimeError: main thread is not in main loop with Matplotlib and Flask

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