matplotlib figures disappearing between show() and savefig()

Question

I've kept a set of references to figures in a dictionary so that I could save them later if desired. I am troubled that the saved figures are blank if invoke a show() command and look at them first. Since the show() command blocks and I am not using a spyder-like interpreter, I have to close the figures before I get to savefig()

figures['myfig_1'] = figure()
...
figures['myfig_n'] = figure()
...

#show() #disabling this makes the problem go away
print "Saving:"
for fig in figures:
   figure(figures[fig].number)
   savefig(fig)
   print "Figure " + str(figures[fig].number) + ": " + fig

The print statement here has given me the indication that the dictionary is still intact, which I think means that I have not lost the figure references (they are still returning meaningful numbers in their .number attribute.)

Another twist I have noticed is that when I have done a similar thing in a class, storing the dictionary as a member and dividing the store and save functions into their own methods, this does not happen. Is there something about the way I am closing the figures or storing the data which is making the figures loose their data?

Answer 1

Generally speaking, in cases like this don't use the interactive matlab-ish state machine interface to matplotlib. It's meant for interactive use.

You're trying to make a figure "active", and creating a new figure instead. It doesn't matter which figure is active, if you just retain the returned figure and/or axis objects and use them directly. (Also, don't use wildcard imports! You will regret it at some later point when you're maintaining your code!)

Just do something like this:

import matplotlib.pyplot as plt
figures = {}

figures['a'] = plt.figure()
ax = figures['a'].add_subplot(111)
ax.plot(range(10), 'ro-')

figures['b'] = plt.figure()
ax = figures['b'].add_subplot(111)
ax.plot(range(10), 'bo-')

plt.show()

for name, fig in figures.iteritems():
    fig.savefig('figure-%s.png' % name)