Re: [Matplotlib-users] Importing pyplot blocks input thread

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

On 2013/08/06 10:08 AM, John McFarland wrote:
> I am trying to write some code that uses an input thread to check for user
> input while another thread is running some calculations (see example below).
> What I have noticed is that just including an import of pyplot (regardless
> of whether it is used by the code or not) causes the call to raw_input in
> the input thread to block the main thread.  It works fine when I don't
> import pyplot.  Importing pyplot is a consequence of other libraries that I
> need to use in the code, so I would like to find a way to make this work.
>
> I have python 2.7.3, matplotlib 1.2.0, and am currently using the GTKAgg
> backend.  Any help is much appreciated.
>
> Here is the example code:
>
> import time
> import threading

Before the *first* import of pyplot, you need to have:

import matplotlib
matplotlib.use("agg")

or specify any other non-interactive backend.  Alternatively, you can 
specify the backend in a matplotlibrc file.

This assumes you don't actually need an interactive backend.  If you do 
need it, then I suspect you will need to change the strategy you are 
using in your program, ideally eliminating the input thread.  You might 
use a gtk idle event callback to handle the user input, for example. 
The problem here is that python threads and gui toolkits tend not to mix 
well.

I suspect that raw_input is using the PyOS_InputHook, which is also 
being used by gtk, so you are violating the prohibition against 
gui-related activities being in more than one thread.

Eric

> import matplotlib.pyplot  # Works fine if this is commented out
>
> def input_thread():
>      raw_input('Press a key:')
>      print "Input data received"
>
> thread = threading.Thread(target=input_thread)
> thread.start()
> time.sleep(.01)
> print
>
> # Main thread (e.g. a calculation that can take some time)
> for i in xrange(10):
>      print i
>      time.sleep(.5)
>
>
>
> --
> View this message in context: http://matplotlib.1069221.n5.nabble.com/Importing-pyplot-blocks-input-thread-tp41731.html
> Sent from the matplotlib - users mailing list archive at Nabble.com.
>
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