warning about too many open figures

Question

In a script where I create many figures with fix  ax   plt subplots       I get the warning RuntimeWarning  More than 20 figures have been opened  Figures created through the pyplot interface  matplotlib pyplot figure  are retained until explicitly closed and may consume too much memory    However  I don t understand why I get this warning  because after saving the figure with fig savefig       I delete it with fig clear    del fig  At no point in my code  I have more than one figure open at a time  Still  I get the warning about too many open figures   What does that mean   how can I avoid getting the warning

User · Answer

import matplotlib pyplot as plt   plt rcParams update   figure max open warning   0    If you use this  you won   t get that error  and it is the simplest way to do that

User · Answer

Use  clf or  cla on your figure object instead of creating a new figure  From  DavidZwicker  Assuming you have imported pyplot as  import matplotlib pyplot as plt   plt cla   clears an axis  i e  the currently active axis in the current figure  It leaves the other axes untouched   plt clf   clears the entire current figure with all its axes  but leaves the window opened  such that it may be reused for other plots   plt close   closes a window  which will be the current window  if not specified otherwise  plt close  all   will close all open figures   The reason that del fig does not work is that the pyplot state-machine keeps a reference to the figure around  as it must if it is going to know what the  current figure  is   This means that even if you delete your ref to the figure  there is at least one live ref  hence it will never be garbage collected   Since I m polling on the collective wisdom here for this answer   JoeKington mentions in the comments that plt close fig  will remove a specific figure instance from the pylab state machine  plt  pylab helpers Gcf  and allow it to be garbage collected

User · Answer

The following snippet solved the issue for me    class FigureWrapper object          Frees underlying figure when it goes out of scope                def   init   self  figure           self  figure   figure      def   del   self           plt close self  figure          print  Figure removed                 f  ax   plt subplots 1  figsize  20  20        wrapped figure   FigureWrapper f       ax plot         plt savefig                When  wrapped figure goes out of scope the runtime calls our   del     method with plt close   inside  It happens even if exception fires after  wrapped figure constructor

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Here s a bit more detail to expand on Hooked s answer  When I first read that answer  I missed the instruction to call clf   instead of creating a new figure  clf   on its own doesn t help if you then go and create another figure   Here s a trivial example that causes the warning   from matplotlib import pyplot as plt  patches import os   def main        path    figures      for i in range 21            fig  ax   plt subplots           x   range 3 i          y    n n for n in x          ax add patch patches Rectangle xy  i  1   width i  height 10           plt step x  y  linewidth 2  where  mid           figname    fig    png  format i          dest   os path join path  figname          plt savefig dest     write image to file         plt clf       print  Done     main     To avoid the warning  I have to pull the call to subplots   outside the loop  In order to keep seeing the rectangles  I need to switch clf   to cla    That clears the axis without removing the axis itself   from matplotlib import pyplot as plt  patches import os   def main        path    figures       fig  ax   plt subplots       for i in range 21           x   range 3 i          y    n n for n in x          ax add patch patches Rectangle xy  i  1   width i  height 10           plt step x  y  linewidth 2  where  mid           figname    fig    png  format i          dest   os path join path  figname          plt savefig dest     write image to file         plt cla       print  Done     main     If you re generating plots in batches  you might have to use both cla   and close    I ran into a problem where a batch could have more than 20 plots without complaining  but it would complain after 20 batches  I fixed that by using cla   after each plot  and close   after each batch   from matplotlib import pyplot as plt  patches import os   def main        for i in range 21           print  Batch     format i           make plots  figures       print  Done      def make plots path       fig  ax   plt subplots       for i in range 21           x   range 3   i          y    n   n for n in x          ax add patch patches Rectangle xy  i  1   width i  height 10           plt step x  y  linewidth 2  where  mid           figname    fig    png  format i          dest   os path join path  figname          plt savefig dest     write image to file         plt cla       plt close fig    main     I measured the performance to see if it was worth reusing the figure within a batch  and this little sample program slowed from 41s to 49s  20  slower  when I just called close   after every plot

User · Answer

This is also useful if you only want to temporarily suppress the warning  import matplotlib pyplot as plt         with plt rc context rc   figure max open warning   0        lots of plots

User · Answer

If you intend to knowingly keep many plots in memory  but don t want to be warned about it  you can update your options prior to generating figures   import matplotlib pyplot as plt plt rcParams update   figure max open warning   0     This will prevent the warning from being emitted without changing anything about the way memory is managed

[python] warning about too many open figures

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