A little background, copied from the PyPy issue:

PyOS_InputHook function is exposed in CPython's pythonrun.h, and documented in https://docs.python.org/3/c-api/veryhigh.html. It is used in CPython's builtin tkinter and readline modules.

PyPy uses a pure-python readline implementation, and its set_pre_input_hook function is not implemented.

The relevant code path in tkinter (EnableEventHook and DisableEventHook) is commented out on PyPy. Other platforms perform properly without the hook, I do not know why the MacOS platform uses it on matplotlib.

My guess is they were needed at one point to allow the tkinter event loop and the REPL event loop to play nicely. But then why were they MacOS only, unless at one point python vendored a broken version of tk on that platform?

Maybe someone with the oldest python and MacOS matplotlib supports could verify the terminal and the canvas are both responsive after removing the callback

Ok away from computer but maybe we should merge this as fixing pypy and then open the rest as a separate issue.

@mdehoon, could you comment?

This is very tricky stuff; the backend may seem to work fine but you may notice problems later.
The fundamental problem here is that Python does not support event loops. PyOS_InputHook allows one C extension (e.g. the MacOSX backend) to run an event loop, but this may break if there are more than one C extensions that need an event loop (e.g. the MacOSX backend and Tkinter). One thing to check for is if the CPU usage goes up to 100%.

Thanks @mdehoon. I checked and we definitely need these hooks for the non pypy work.