Why matplotlib requires to plot only in the main thread?

Question

I\'m trying to plot live the output of a generator.

The following code works as expected (Ctrl-C terminates execution):

import numpy as np
import pyl         


        

Answer 1

It's probably the GUI that you're using for backend. The GUI likely expects to find itself in the main thread, but it isn't when matplotlib calls get_current_fig_manager().canvas.draw().

For example, when I do this, I get the following traceback:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "tmp.py", line 18, in plotter
    p.draw()
  File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 555, in draw
    get_current_fig_manager().canvas.draw()
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 349, in draw
    tkagg.blit(self._tkphoto, self.renderer._renderer, colormode=2)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/tkagg.py", line 13, in blit
    tk.call("PyAggImagePhoto", photoimage, id(aggimage), colormode, id(bbox_array))
RuntimeError: main thread is not in main loop

Note the tk.call(...) line. The exception you get is not raised from matplotlib, it's raised from TkInter.