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Matplotlib deprecation warning #183
Description
see, e.g.,
https://travis-ci.org/python-control/python-control/jobs/325460249#L2815
The warning says:
test_siso (control.tests.freqresp_test.TestFreqresp) ... /home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/matplotlib/cbook/deprecation.py:106: 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.
Activity
murrayrm commented on Jan 15, 2018
This has been fixed in PR #187. The issue is a planned change in the behavior of the matplotlib axes command (which is called by matplotlib.subplot in control.bode_plot and control.gangof4_plot). In the old behavior, if you called axes or subplot with the same argument, you got a pointer to the previous object. This behavior is not desired and so it is being deprecated. Details in matplotlib PR #9037.
What generated this behavior in the test suit was that we called bode twice in a row in the unit test and this caused matplotlib.pyplot.subfig to get called with the same arguments. The fix was to add a randomly generated internal "label" (not displayed anywhere), as described in the warning message.