Improve warning message for missing font family specified via alias.

anntzer · anntzer · commit 3f4172ade9d7 · 2020-11-11T09:18:27.000+01:00
See example of warning message in the test.
diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
@@ -1113,6 +1113,12 @@ def set_default_weight(self, weight):
         """
         self.__default_weight = weight
 
+    @staticmethod
+    def _expand_aliases(family):
+        if family in ('sans', 'sans serif'):
+            family = 'sans-serif'
+        return rcParams['font.' + family]
+
     # Each of the scoring functions below should return a value between
     # 0.0 (perfect match) and 1.0 (terrible match)
     def score_family(self, families, family2):
@@ -1135,10 +1141,7 @@ def score_family(self, families, family2):
         for i, family1 in enumerate(families):
             family1 = family1.lower()
             if family1 in font_family_aliases:
-                if family1 in ('sans', 'sans serif'):
-                    family1 = 'sans-serif'
-                options = rcParams['font.' + family1]
-                options = [x.lower() for x in options]
+                options = [*map(str.lower, self._expand_aliases(family1))]
                 if family2 in options:
                     idx = options.index(family2)
                     return (i + (idx / len(options))) * step
@@ -1341,6 +1344,12 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                 _log.warning(
                     'findfont: Font family %s not found. Falling back to %s.',
                     prop.get_family(), self.defaultFamily[fontext])
+                for family in map(str.lower, prop.get_family()):
+                    if family in font_family_aliases:
+                        _log.warning(
+                            "findfont: Generic family %r not found because "
+                            "none of the following families were found: %s",
+                            family, ", ".join(self._expand_aliases(family)))
                 default_prop = prop.copy()
                 default_prop.set_family(self.defaultFamily[fontext])
                 return self.findfont(default_prop, fontext, directory,
diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
@@ -210,3 +210,15 @@ def test_fork():
     ctx = multiprocessing.get_context("fork")
     with ctx.Pool(processes=2) as pool:
         pool.map(_model_handler, range(2))
+
+
+def test_missing_family(caplog):
+    plt.rcParams["font.sans-serif"] = ["this-font-does-not-exist"]
+    with caplog.at_level("WARNING"):
+        findfont("sans")
+    assert [rec.getMessage() for rec in caplog.records] == [
+        "findfont: Font family ['sans'] not found. "
+        "Falling back to DejaVu Sans.",
+        "findfont: Generic family 'sans' not found because none of the "
+        "following families were found: this-font-does-not-exist",
+    ]
