matplotlib plotting freezing in simple thread

Question

Been playing with plotting libraries for Python and came across matplotlib which seems to be battle tested and proven already. However I have came across a problem while creating a simple plot in a thread.

In the example bellow, Dummy class plotme method is run in a thread twice in a row, but it gets stuck/freezes in 2nd iteration. Most likely is something obvious and related to the threading itself, but I failed to spot it so far.

import matplotlib.pyplot as plt
from numpy import arange, sin, pi
import threading

class Dummy():

    def plotme(self, iteration = 1):

        print "%ix plotting... " % iteration,
        t = arange(0.0, 2.0, 0.01)
        s = sin(2*pi*t)
        plt.plot(t, s)
        plt.xlabel('time (s)')
        plt.ylabel('voltage (mV)')
        plt.title('About as simple as it gets, folks')
        #savefig("test.png") # irrelevant here
        plt.close()

    def threadme(self, iteration = 1):

        thread_plot = threading.Thread(target=self.plotme,
                                      args=(iteration,))
        thread_plot.start()
        thread_plot.join()

dummy = Dummy()
dummy.threadme(1)
dummy.threadme(2)

Are you aware that the commands in matplotlib.pyplot are not hread-safe? Your should use an OOÜP-approach, like creating a figure, an axes-object and then calling the methods on this axes-object, e.g. ax.plot(...) — Thorsten Kranz
– Thorsten Kranz, Commented Oct 1, 2013 at 8:10

Thorsten Kranz · Accepted Answer · 2013-10-01 08:16:48Z

5

First, be aware that pyplot-interface is not thread-safe.

Then: use the "Agg"-backend for non-interactive creation of multiple images.

A working example (with possible problems due to threading) is:

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
from numpy import arange, sin, pi
import threading

class Dummy():

    def plotme(self, iteration = 1):

        print "%ix plotting... " % iteration,
        t = arange(0.0, 2.0, 0.01)
        s = sin(2*pi*t)
        plt.plot(t, s)
        plt.xlabel('time (s)')
        plt.ylabel('voltage (mV)')
        plt.title('About as simple as it gets, folks')
        plt.savefig("19110942_%i_test.png" % iteration) # irrelevant here
        plt.clf()

    def threadme(self, iteration = 1):

        thread_plot = threading.Thread(target=self.plotme,
                                      args=(iteration,))
        thread_plot.start()
        thread_plot.join()

dummy = Dummy()
dummy.threadme(1)
dummy.threadme(2)

A thread-safe version would look like this:

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
from numpy import arange, sin, pi
import threading

class Dummy():

    def plotme(self, iteration = 1):

        print "%ix plotting... " % iteration,
        t = arange(0.0, 2.0, 0.01)
        s = sin(2*pi*t)

        fig, ax = plt.subplots()
        ax.plot(t, s)
        ax.set_xlabel('time (s)')
        ax.set_ylabel('voltage (mV)')
        ax.set_title('About as simple as it gets, folks (%i)' % iteration)
        fig.savefig("19110942_%i_test.png" % iteration)

    def threadme(self, iteration = 1):

        thread_plot = threading.Thread(target=self.plotme,
                                      args=(iteration,))
        thread_plot.start()
        thread_plot.join()

dummy = Dummy()
dummy.threadme(1)
dummy.threadme(2)

answered Oct 1, 2013 at 8:16

Thorsten Kranz

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2 Comments

Luis Mendes Over a year ago

Thanks @Thorsten Kranz that's it. I should have investigated properly about pyplot "thread-safeness" :)

SiHa Over a year ago

Note: I have been playing with this over the last couple of days, and can say with some confidence that the second version above is no more 'thread-safe' than the first, both will happily hang on subsequent threaded runs, if matplotlib.use('Agg') is not called. The key is using non-interactive mode, and then both will happily run many, many times in a thread without error.

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