Plotting in a non-blocking way with Matplotlib

Question

I have been playing with Numpy and matplotlib in the last few days  I am having problems trying to make matplotlib plot a function without blocking execution  I know there are already many threads here on SO asking similar questions  and I  ve googled quite a lot but haven t managed to make this work  I have tried using show block False  as some people suggest  but all I get is a frozen window  If I simply call show    the result is plotted properly but execution is blocked until the window is closed  From other threads I  ve read  I suspect that whether show block False  works or not depends on the backend  Is this correct  My back end is Qt4Agg  Could you have a look at my code and tell me if you see something wrong  Here is my code  Thanks for any help  from math import   from matplotlib import pyplot as plt print plt get backend       def main        x   range -50  51  1      for pow in range 1 5       plot x 1  x 2       x 4          y    Xi  pow for Xi in x          print y           plt plot x  y          plt draw            plt show                this plots correctly  but blocks execution          plt show block False     this creates an empty frozen window              raw input  quot Press  enter  to continue  quot     if   name         main         main    PS  I forgot to say that I would like to update the existing window every time I plot something  instead of creating a new one

User · Answer

Live Plotting  import numpy as np import matplotlib pyplot as plt  x   np linspace 0  2   np pi  100    plt axis  x 0   x -1   -1  1          disable autoscaling for point in x      plt plot point  np sin 2   point        color  b       plt draw       plt pause 0 01    plt clf                               clear the current figure   if the amount of data is too much you can lower the update rate with a simple counter  cnt    1 if  cnt    10           update plot each 10 points     plt draw       plt pause 0 01      cnt   0   Holding Plot after Program Exit  This was my actual problem that couldn t find satisfactory answer for  I wanted plotting that didn t close after the script was finished  like MATLAB     If you think about it  after the script is finished  the program is terminated and there is no logical way to hold the plot this way  so there are two options   block the script from exiting  that s plt show   and not what I want  run the plot on a separate thread  too complicated    this wasn t satisfactory for me so I found another solution outside of the box  SaveToFile and View in external viewer  For this the saving and viewing should be both fast and the viewer shouldn t lock the file and should update the content automatically  Selecting Format for Saving  vector based formats are both small and fast   SVG is good but coudn t find good viewer for it except the web browser which by default needs manual refresh PDF can support vector formats and there are lightweight viewers which support live updating   Fast Lightweight Viewer with Live Update  For PDF there are several good options   On Windows I use SumatraPDF which is free  fast and light  only uses 1 8MB RAM for my case  On Linux there are several options such as Evince  GNOME  and Ocular  KDE    Sample Code  amp  Results  Sample code for outputing plot to a file  import numpy as np import matplotlib pyplot as plt  x   np linspace 0  2   np pi  100  y   np sin 2   x  plt plot x  y  plt savefig  fig pdf     after first run  open the output file in one of the viewers mentioned above and enjoy   Here is a screenshot of VSCode alongside SumatraPDF  also the process is fast enough to get semi-live update rate  I can get near 10Hz on my setup just use time sleep   between intervals

User · Answer

You can avoid blocking execution by writing the plot to an array  then displaying the array in a different thread  Here is an example of generating and displaying plots simultaneously using pf screen from pyformulas 0 2 8   import pyformulas as pf import matplotlib pyplot as plt import numpy as np import time  fig   plt figure    canvas   np zeros  480 640   screen   pf screen canvas   Sinusoid    start   time time   while True      now   time time   - start      x   np linspace now-2  now  100      y   np sin 2 np pi x    np sin 3 np pi x      plt xlim now-2 now 1      plt ylim -3 3      plt plot x  y  c  black          If we haven t already shown or saved the plot  then we need to draw the figure first        fig canvas draw        image   np fromstring fig canvas tostring rgb    dtype np uint8  sep         image   image reshape fig canvas get width height     -1     3         screen update image    screen close     Result     Disclaimer  I m the maintainer for pyformulas   Reference  Matplotlib  save plot to numpy array

User · Answer

I spent a long time looking for solutions  and found this answer   It looks like  in order to get what you  and I  want  you need the combination of plt ion    plt show    not with block False  and  most importantly  plt pause  001   or whatever time you want   The pause is needed because the GUI events happen while the main code is sleeping  including drawing  It s possible that this is implemented by picking up time from a sleeping thread  so maybe IDEs mess with that   I don t know   Here s an implementation that works for me on python 3 5    import numpy as np from matplotlib import pyplot as plt  def main        plt axis  -50 50 0 10000       plt ion       plt show        x   np arange -50  51      for pow in range 1 5       plot x 1  x 2       x 4         y    Xi  pow for Xi in x          plt plot x  y          plt draw           plt pause 0 001          input  Press  enter  to continue     if   name         main         main

User · Answer

I figured out that the plt pause 0 001  command is the only thing needed and nothing else  plt show   and plt draw   are unnecessary and   or blocking in one way or the other  So here is a code that draws and updates a figure and keeps going  Essentially plt pause 0 001  seems to be the closest equivalent to matlab s drawnow  Unfortunately those plots will not be interactive  they freeze   except you insert an input   command  but then the code will stop  The documentation of the plt pause interval  command states   If there is an active figure  it will be updated and displayed before the pause       This can be used for crude animation   and this is pretty much exactly what we want  Try this code  import numpy as np from matplotlib import pyplot as plt  x   np arange 0  51                  x coordinates             for z in range 10  50        y   np power x  z 10             y coordinates of plot for animation      plt cla                          delete previous plot     plt axis  -50  50  0  10000      set axis limits  to avoid rescaling     plt plot x  y                    generate new plot     plt pause 0 1                    pause 0 1 sec  to force a plot redraw

User · Answer

Iggy s answer was the easiest for me to follow  but I got the following error when doing a subsequent subplot command that was not there when I was just doing show      MatplotlibDeprecationWarning  Adding an axes using the same arguments   as a previous axes currently reuses the earlier instance   In a future   version  a new instance will always be created and returned     Meanwhile  this warning can be suppressed  and the future behavior   ensured  by passing a unique label to each axes instance    In order to avoid this error  it helps to close  or clear  the plot after the user hits enter   Here s the code that worked for me   def plt show           Text-blocking version of plt show       Use this instead of plt show          plt draw       plt pause 0 001      input  Press enter to continue          plt close

User · Answer

A lot of these answers are super inflated and from what I can find  the answer isn t all that difficult to understand   You can use plt ion   if you want  but I found using plt draw   just as effective  For my specific project I m plotting images  but you can use plot   or scatter   or whatever instead of figimage    it doesn t matter   plt figimage image to show  plt draw   plt pause 0 001    Or  fig   plt figure       fig figimage image to show  fig canvas draw   plt pause 0 001    If you re using an actual figure  I used  krs013  and  Default Picture s answers to figure this out Hopefully this saves someone from having launch every single figure on a separate thread  or from having to read these novels just to figure this out

User · Answer

A simple trick that works for me is the following    Use the block   False argument inside show  plt show block   False  Use another plt show   at the end of the  py script      Example   import matplotlib pyplot as plt  plt imshow add something  plt xlabel  x   plt ylabel  y    plt show block False    more code here  e g  do calculations and use print to see them on the screen  plt show       Note  plt show   is the last line of my script

User · Answer

The Python package drawnow allows to update a plot in real time in a non blocking way  It also works with a webcam and OpenCV for example to plot measures for each frame  See the original post

[python] Plotting in a non-blocking way with Matplotlib

Live Plotting

Holding Plot after Program Exit

SaveToFile and View in external viewer

Selecting Format for Saving

Fast Lightweight Viewer with Live Update

Sample Code & Results

Examples related to python

Examples related to matplotlib

Examples related to plot