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

Why does matplotlib care which thread does the plotting? I'm not asking how to solve this but rather why is this happening in the first place.

@Evert is right, it's not matplotlib, it's your GUI toolkit (one of the backends that matplotlib uses to create a window with a plot for you). It happens because GUI toolkits are event-driven (you don't want blocking behavior for the user interface, right?) and they have internal event loop, that controls program execution. The idea is that events are monitored by the event loop and dispatched to the callbacks. To do this, event loop should be started in the main thread, while callbacks for long-running tasks are moved to separate threads.

Answer 2

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.