python - 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?

Answer 1

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.