Add abstract methods and ABC on Fonts classes

mypy rightfully complains that these methods called in the base class
don't exist, so correctly indicate what should be implemented there.

The ABC is not strictly necessary, but it helps with understanding as a
lot of these have code that makes them seem like _not_ an ABC.

Author: QuLogic

lib/matplotlib/_mathtext.py

@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+import abc
 import copy
 import enum
 import functools
@@ -216,7 +217,7 @@ class FontInfo(NamedTuple):
     offset: float
 
 
-class Fonts:
+class Fonts(abc.ABC):
     """
     An abstract base class for a system of fonts to use for mathtext.
 
@@ -248,6 +249,13 @@ def get_kern(self, font1, fontclass1, sym1, fontsize1,
         """
         return 0.
 
+    def _get_font(self, font: str) -> FT2Font:
+        raise NotImplementedError
+
+    def _get_info(self, font: str, font_class: str, sym: str, fontsize: float,
+                  dpi: float) -> FontInfo:
+        raise NotImplementedError
+
     def get_metrics(self, font, font_class, sym, fontsize, dpi):
         r"""
         Parameters
@@ -313,7 +321,7 @@ def get_sized_alternatives_for_symbol(self, fontname, sym):
         return [(fontname, sym)]
 
 
-class TruetypeFonts(Fonts):
+class TruetypeFonts(Fonts, metaclass=abc.ABCMeta):
     """
     A generic base class for all font setups that use Truetype fonts
     (through FT2Font).
@@ -324,6 +332,7 @@ def __init__(self, *args, **kwargs):
         # Per-instance cache.
         self._get_info = functools.cache(self._get_info)
         self._fonts = {}
+        self.fontmap: dict[str | int, str] = {}
 
         filename = findfont(self.default_font_prop)
         default_font = get_font(filename)
@@ -348,6 +357,10 @@ def _get_offset(self, font, glyph, fontsize, dpi):
             return (glyph.height / 64 / 2) + (fontsize/3 * dpi/72)
         return 0.
 
+    def _get_glyph(self, fontname: str, font_class: str,
+                   sym: str) -> tuple[FT2Font, int, bool]:
+        raise NotImplementedError
+
     # The return value of _get_info is cached per-instance.
     def _get_info(self, fontname, font_class, sym, fontsize, dpi):
         font, num, slanted = self._get_glyph(fontname, font_class, sym)
@@ -429,7 +442,6 @@ def __init__(self, *args, **kwargs):
         self._stix_fallback = StixFonts(*args, **kwargs)
 
         super().__init__(*args, **kwargs)
-        self.fontmap = {}
         for key, val in self._fontmap.items():
             fullpath = findfont(val)
             self.fontmap[key] = fullpath
@@ -543,7 +555,6 @@ def __init__(self, *args, **kwargs):
         self._fallback_font = font_cls(*args, **kwargs) if font_cls else None
 
         super().__init__(*args, **kwargs)
-        self.fontmap = {}
         for texfont in "cal rm tt it bf sf bfit".split():
             prop = mpl.rcParams['mathtext.' + texfont]
             font = findfont(prop)
@@ -641,7 +652,8 @@ def get_sized_alternatives_for_symbol(self, fontname, sym):
         return [(fontname, sym)]
 
 
-class DejaVuFonts(UnicodeFonts):
+class DejaVuFonts(UnicodeFonts, metaclass=abc.ABCMeta):
+    _fontmap: dict[str | int, str] = {}
 
     def __init__(self, *args, **kwargs):
         # This must come first so the backend's owner is set correctly
@@ -651,7 +663,6 @@ def __init__(self, *args, **kwargs):
             self._fallback_font = StixSansFonts(*args, **kwargs)
         self.bakoma = BakomaFonts(*args, **kwargs)
         TruetypeFonts.__init__(self, *args, **kwargs)
-        self.fontmap = {}
         # Include Stix sized alternatives for glyphs
         self._fontmap.update({
             1: 'STIXSizeOneSym',
@@ -749,7 +760,6 @@ class StixFonts(UnicodeFonts):
 
     def __init__(self, *args, **kwargs):
         TruetypeFonts.__init__(self, *args, **kwargs)
-        self.fontmap = {}
         for key, name in self._fontmap.items():
             fullpath = findfont(name)
             self.fontmap[key] = fullpath