Summary

Backends can provide a draw_if_interactive function, which, if present, gets used as the implementation of plt.draw_if_interactive. If not, the default behavior of that function is to do nothing in noninteractive; if in interactive mode, it calls draw_idle() on the active canvas. The only places where Matplotlib calls draw_if_interactive itself is at the end of figure() and of figure unpickling, to trigger a first draw. Prior to the introduction of Artist.stale, it was also called at the end of each pyplot plotting method, but this is now handled by the stale callback (which calls draw_idle(), and is not overridden by a backend-provided draw_if_interactive; however, backends could still register their own stale callback).

As part of my "move everything backend-related to FigureCanvas" work, I would like to deprecate per-backend customization of draw_if_interactive (i.e. plt.draw_if_interactive would remain present, but always just be "if in interactive mode, call draw_idle() on the active canvas"). No builtin backend actually relies on this mechanism, but there's at least ipympl and matplotlib-inline who do customize it in non-trivial ways; however, they could move their custom logic to draw_idle() instead (keeping track of whether this is the first call to draw_idle). I would like to know if there's opposition to such a change.

It would be slightly annoying if we said e.g. "if your backend provides a draw_if_interactive, then emit a DeprecationWarning", because this would make it impossible to write third-party backends that work both with the old and the new backend APIs (as defined in #23101). However, we can take advantage of the introduction of the new backend API to say "if you want to move to the new API, you must do it all at once; specifically, if you don't provide a new_figure_manager function, then we won't look for a draw_if_interactive function either". To do so, this behavior needs to be introduced in 3.6 (at the same time as the new backend API); hence, I open this issue mostly to add this to the 3.6 milestone.

Proposed fix

No response