Log a warning if selected font weight differs from requested

Author: Colton Lathrop

doc/api/next_api_changes/behavior/30272-ES.rst

@@ -0,0 +1,2 @@
+``font_manager.findfont`` logs if selected font weight does not match requested
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

lib/matplotlib/font_manager.py

@@ -172,6 +172,10 @@
 ]
 
 
+def _normalize_weight(weight):
+    return weight if isinstance(weight, Number) else weight_dict[weight]
+
+
 def get_fontext_synonyms(fontext):
     """
     Return a list of file extensions that are synonyms for
@@ -1256,8 +1260,8 @@ def score_weight(self, weight1, weight2):
         # exact match of the weight names, e.g. weight1 == weight2 == "regular"
         if cbook._str_equal(weight1, weight2):
             return 0.0
-        w1 = weight1 if isinstance(weight1, Number) else weight_dict[weight1]
-        w2 = weight2 if isinstance(weight2, Number) else weight_dict[weight2]
+        w1 = _normalize_weight(weight1)
+        w2 = _normalize_weight(weight2)
         return 0.95 * (abs(w1 - w2) / 1000) + 0.05
 
     def score_size(self, size1, size2):
@@ -1480,6 +1484,11 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                 best_font = font
             if score == 0:
                 break
+        if best_font is not None:
+            if (_normalize_weight(prop.get_weight()) !=
+                    _normalize_weight(best_font.weight)):
+                _log.warning('findfont: Failed to find font weight %s, now using %s.',
+                             prop.get_weight(), best_font.weight)
 
         if best_font is None or best_score >= 10.0:
             if fallback_to_default:

lib/matplotlib/font_manager.pyi

@@ -1,14 +1,13 @@
+from collections.abc import Iterable
 from dataclasses import dataclass
+from numbers import Number
 import os
+from pathlib import Path
+from typing import Any, Literal
 
 from matplotlib._afm import AFM
 from matplotlib import ft2font
 
-from pathlib import Path
-
-from collections.abc import Iterable
-from typing import Any, Literal
-
 font_scalings: dict[str | None, float]
 stretch_dict: dict[str, int]
 weight_dict: dict[str, int]
@@ -19,6 +18,7 @@ MSUserFontDirectories: list[str]
 X11FontDirectories: list[str]
 OSXFontDirectories: list[str]
 
+def _normalize_weight(weight: str | Number) -> Number: ...
 def get_fontext_synonyms(fontext: str) -> list[str]: ...
 def list_fonts(directory: str, extensions: Iterable[str]) -> list[str]: ...
 def win32FontDirectory() -> str: ...

lib/matplotlib/tests/test_font_manager.py

@@ -15,7 +15,8 @@
 from matplotlib.font_manager import (
     findfont, findSystemFonts, FontEntry, FontProperties, fontManager,
     json_dump, json_load, get_font, is_opentype_cff_font,
-    MSUserFontDirectories, _get_fontconfig_fonts, ttfFontProperty)
+    MSUserFontDirectories, ttfFontProperty,
+    _get_fontconfig_fonts, _normalize_weight)
 from matplotlib import cbook, ft2font, pyplot as plt, rc_context, figure as mfigure
 from matplotlib.testing import subprocess_run_helper, subprocess_run_for_testing
 
@@ -407,3 +408,26 @@ def test_fontproperties_init_deprecation():
     # Since this case is not covered by docs, I've refrained from jumping
     # extra hoops to detect this possible API misuse.
     FontProperties(family="serif-24:style=oblique:weight=bold")
+
+
+def test_map_weights():
+    assert _normalize_weight('ultralight') == 100
+    assert _normalize_weight('light') == 200
+    assert _normalize_weight('normal') == 400
+    assert _normalize_weight('regular') == 400
+    assert _normalize_weight('book') == 400
+    assert _normalize_weight('medium') == 500
+    assert _normalize_weight('roman') == 500
+    assert _normalize_weight('semibold') == 600
+    assert _normalize_weight('demibold') == 600
+    assert _normalize_weight('demi') == 600
+    assert _normalize_weight('bold') == 700
+    assert _normalize_weight('heavy') == 800
+    assert _normalize_weight('extra bold') == 800
+    assert _normalize_weight('black') == 900
+
+
+def test_font_match_warning(caplog):
+    findfont(FontProperties(family=["DejaVu Sans"], weight=750))
+    logs = [rec.message for rec in caplog.records]
+    assert 'findfont: Failed to find font weight 750, now using 700.' in logs