Summary

Currently, FontProperties (FPs) are mutable, which makes caching based on them a bit tricky (#22323,#22487). Manipulation is also not so nice; e.g. the fonts_demo.py example needs to repeatedly copy a base FP instance and call setters on the copies. Moreover, if one writes

fp = FontProperties(...)textobj.set_fontproperties(fp)

then later mutation offp doesn't modify the fontproperties ontextobj, becauseset_fontproperties explicitly makes a copy of its argument. While this kind of "isolate-from-later-changes" is something we typically aim for in Matplotlib (see recent PRs likewise isolating artists from later changes in passed-in data arrays), this also means that there is basically no functionality gain coming from mutable FPs (and at least one complication, in the case of caching). (Ifset_fontproperties did not copy, then mutability would provide a way for two text objects to share exactly the same FP while having to mutate only one of them.)

I would therefore argue that the various property setters on FP should be deprecated to ultimately make FPs immutable; instead, we should usewith_foo() methods that return new instances with just a single property being changed (similar to the pathlib API, for example). More precisely, we can either havewith_family(family),with_style(style),with_variant(variant), etc., or instead we could also have a singlewith_props(**kwargs) (called e.g. aswith_props(family="Times", ...)); I don't have a very strong opinion there.

Proposed fix

No response