Re: [Matplotlib-users] On GtkAgg, one draw bad, two draws good?

[Eric F. <ef...@ha...>]

On 02/25/2012 03:54 AM, Alejandro Dubrovsky wrote:
> Hi,
>
> I've run into something that I expect to be ground well covered but I
> can't find anything relevant on google.
>
> I'm on linux. When using the GtkAgg backend on matplotlib 1.1.0 (or git
> head for that matter), with interactive mode on, one call to draw() only
> gives me a blank window, while a second draw() immediately afterwards
> will draw the requested graph. On IRC, askewchan told me it only one
> draw is required under MacOSX, and it also works fine with just one draw
> call if I switch to the TkAgg backend.

This does not have a simple answer.  The basic problem is a collision 
between event-driven programming (the gui toolkit) and nice, simple 
sequential programming such as in your script.  The plotting commands 
essentially register requests with the gui toolkit, and the toolkit 
executes them when it gets around to it.  It won't do it during a 
time.sleep().  Typically, it has to be waiting for input, because that 
is when the mainloop is checking to see if there is anything that needs 
to be done.

>
> The following short script should demonstrate the issue (note: this is
> only a problem when run as a script. It works fine on ipython, or
> standard python prompt):
>

Try removing *all* your "draw" commands, and replace the calls to 
time.sleep(2) with plt.pause(2).  That works for me on gtkagg, qt4agg, 
wxagg, and tkagg. plt.pause() calls the event loop while waiting, 
instead of just freezing everything like sleep() does.

Eric

> ---
> import time
> from matplotlib import pyplot as plt
>
> def graph():
>
> 	plt.ion()
>
> 	plt.plot(range(10), range(10,20))
> 	plt.draw()
> 	plt.draw()  # blank screen if this omitted
> 	time.sleep(2)
> 	plt.plot(range(30,40), range(10,20))
> 	plt.draw()
> 	plt.draw()  # second line not drawn if this omitted
> 	time.sleep(2)
>
> 	plt.ioff()
>
> graph()
> ---
>
> backend GTKAgg version 2.24.0
> As a different but most likely related issue, th Qt4Agg backend doesn't
> work even with two draw calls.
>
> Is this a known issue? Is the script not using matplotlib as expected?
> If so, is running a separate thread the intended way of showing graphs
> in a non-blocking way?
>
> Thanks,
> alejandro
>
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