I just noticed that the wheels on PyPI still set a default backend in their matplotlibrc (to tkagg on linux/windows and to macosx on macos). I think this should change, and we should instead omit the default backend and trigger fallback.

Currently, a linux/windows user who installs matplotlib from pypi and does not bother writing a matplotlibrc always gets tkagg, whereas backend fallback is triggered if (and only if) they explicitly write a matplotlibrc and explicitly ignore the backend: ... entry in it. This seems backwards: I'd say backend fallback is here to help the "unconfigured" case, more than the "you have a config but ignored a key" case. This also invalidates the relevant 3.0 whatsnew entry (https://matplotlib.org/users/whats_new.html#improved-default-backend-selection) in the default case of a PyPI install.

(Also, setting the default backend to macosx on macos is not really helpful -- backend fallback will already choose to use that backend if that works, and, if it doesn't (assuming nonframework builds are still an issue, which I'm not certain about), will perform the required fallback.)

If downstream redistributors want to set a default backend (#11844) that's their problem, but I think setting it on the PyPI wheels defeats some of the purpose of having backend fallback to start with...

Matplotlib version

• Operating system: any
• Matplotlib version: 3.0+
• Matplotlib backend (print(matplotlib.get_backend())): that's the question
• Python version: any