We can close #19602 if simpler, though suggest stealing some of the commentary. Also suggest the softlink so no one complains...

More detail:

Previously matplotlibrc.template was changed in #15029 to be the canonical place to get the defaults for all the rcParams. However, these were read during __init__.py from lib/matplotlib/mpl-data/matplotlibrc. lib/matplotlib/mpl-data/matplotlibrc doesn't exist in the repo, and only gets created when setup.py is called. We can't read the defaults from matplotlbrc.template because that file probably doesn't exist in most user installs.

Aside from being somewhat rickety, the problem is that developers working from an editable install don't want to always call setup.py as it requires compiling the C extensions, and hence their lib/matplotlib/mpl-data/matplotlibrc can get out of sync with the library.

The solution here is to simply make lib/matplotlib/mpl-data/matplotlibrc the canonical source of the defaults, add it to version control, (maybe softlink it to matplotlib.template). setup.py can change lib/matplotlib/mpl-data/matplotlibrc in-place

See also:

• Backend related issues with matplotlib 3.0.0rc1 #11844
• Get default params from matplotlibrc.template. #14929
• Get default params from matplotlibrc.template. #15029

added a test checking the default backend.