Python plt: close or clear figure does not work

Question

I generate a lots of figures with a script which I do not display but store to harddrive. After a while I get the message

/usr/lib/pymodules/python2.7/matplotlib/pyplot.py:412: 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. (To control this warning, see the rcParam figure.max_num_figures). max_open_warning, RuntimeWarning)

Thus, I tried to close or clear the figures after storing. So far, I tried all of the followings but no one works. I still get the message from above.

plt.figure().clf()
plt.figure().clear()
plt.clf()
plt.close()
plt.close('all')
plt.close(plt.figure())

And furthermore I tried to restrict the number of open figures by

plt.rcParams.update({'figure.max_num_figures':1})

Here follows a piece of sample code that behaves like described above. I added the different options I tried as comments at the places I tried them.

from pandas import DataFrame
from numpy import random
df = DataFrame(random.randint(0,10,40))

import matplotlib.pyplot as plt
plt.ioff()
#plt.rcParams.update({'figure.max_num_figures':1})
for i in range(0,30):
    fig, ax = plt.subplots()
    ax.hist([df])
    plt.savefig("/home/userXYZ/Development/pic_test.png")
    #plt.figure().clf()
    #plt.figure().clear()
    #plt.clf()
    #plt.close() # results in an error
    #plt.close('all') # also error
    #plt.close(plt.figure()) # also error

To be complete, that is the error I get when using plt.close:

can't invoke "event" command: application has been destroyed while executing "event generate $w <>" (procedure "ttk::ThemeChanged" line 6) invoked from within "ttk::ThemeChanged"

Answer 1

The correct way to close your figures would be to use plt.close(fig), as can be seen in the below edit of the code you originally posted.

from pandas import DataFrame
from numpy import random
df = DataFrame(random.randint(0,10,40))

import matplotlib.pyplot as plt
plt.ioff()
for i in range(0,30):
    fig, ax = plt.subplots()
    ax.hist(df)        
    name = 'fig'+str(i)+'.png'  # Note that the name should change dynamically
    plt.savefig(name)
    plt.close(fig)              # <-- use this line

The error that you describe at the end of your question suggests to me that your problem is not with matplotlib, but rather with another part of your code (such as ttk).

Answer 2

This does not really solve my problem, but it is a work-around to handle the high memory consumption I faced and I do not get any of the error messages as before:

from pandas import DataFrame
from numpy import random
df = DataFrame(random.randint(0,10,40))

import matplotlib.pyplot as plt
plt.ioff()
for i in range(0,30):
    plt.close('all')
    fig, ax = plt.subplots()
    ax.hist([df])
    plt.savefig("/home/userXYZ/Development/pic_test.png")

Answer 3

plt.show() is a blocking function, so in the above code, plt.close() will not execute until the fig windows are closed. You can use plt.ion() at the beginning of your code to make it non-blocking. Even though this has some other implications the fig will be closed.

Answer 4

I was still having the same issue on Python 3.9.7, matplotlib 3.5.1, and VS Code (the issue that no combination of plt.close() closes the figure). I have three loops which the most inner loop plots more than 20 figures. The solution that is working for me is using agg as backend and del someFig after plt.close(someFig). Subsequently, the order of code would be something like:

import matplotlib
matplotlib.use('agg')

import matplotlib.pyplot as plt

someFig = plt.figure()

.
.
.

someFig.savefig('OUTPUT_PATH')
plt.close(someFig) # --> (Note 1)
del someFig

.
.
.

---

• NOTE 1: If this line is removed, the output figures may not be plotted correctly! Especially when the number of elements to be rendered in the figure is high.
• NOTE 2: I don't know whether this solution could backfire or not, but at least it is working and not hugging RAM or preventing plotting figures!