Bug report

Bug summary

draw_event currently fires after the draw has completed (it is triggered at the end of Figure.draw()). This means it cannot modify the Figure, unless one is willing to trigger a second draw -- and even then one needs to be careful to not recurse infinitely. For example, in https://matplotlib.org/3.1.0/gallery/pyplots/auto_subplots_adjust.html (linking the 3.1 version because there's an unrelated bug in 3.2, per #17167), note the call to figure.canvas.draw() at the end of the callback, effectively duplicating the draw(); moreover, if that draw() is moved out of the if block then one gets an infinite recursion. Being able to register a callback to be executed before draw() occurs would avoid both problems.
Likewise a pre-draw callback would provide a very general solution for #17052, by allowing one to simply update the colorbar at pre-draw time.

The same feature was rejected because there wasn't a compelling use case in #5042, but I think I exposed more use cases here. See also #10334 re: recursive redraws.

Matplotlib version

• Operating system:
• Matplotlib version: 3.2
• Matplotlib backend (print(matplotlib.get_backend())):
• Python version:
• Jupyter version (if applicable):
• Other libraries: