Make lack of support more explicit for non-postscript fonts + usetex.

The info-level log can easily be hidden, and the KeyError may look like
a bug on Matplotlib's side.  Instead, explicitly raise an exception
whose error message states that Matplotlib doesn't currently support
certain cases.

Use a LookupError (a parent class of KeyError, so a slight API break)
rather than a KeyError as KeyError formats its argument as if it was the
missing key (calling repr on it), which would not be the case here.

Author: anntzer

doc/api/next_api_changes/deprecations/20474-AL.rst

@@ -0,0 +1,2 @@
+``dviread.PsfontsMap`` now raises LookupError instead of KeyError for missing fonts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

lib/matplotlib/dviread.py

@@ -27,7 +27,6 @@
 import struct
 import subprocess
 import sys
-import textwrap
 
 import numpy as np
 
@@ -852,14 +851,12 @@ def __getitem__(self, texname):
         try:
             return self._parsed[texname]
         except KeyError:
-            fmt = ('An associated PostScript font (required by Matplotlib) '
-                   'could not be found for TeX font {0!r} in {1!r}.  This '
-                   'problem can often be solved by installing a suitable '
-                   'PostScript font package in your TeX package manager.')
-            _log.info(textwrap.fill(
-                fmt.format(texname.decode('ascii'), self._filename),
-                break_on_hyphens=False, break_long_words=False))
-            raise
+            raise LookupError(
+                f"An associated PostScript font (required by Matplotlib) "
+                f"could not be found for TeX font {texname.decode('ascii')!r} "
+                f"in {self._filename!r}; this problem can often be solved by "
+                f"installing a suitable PostScript font package in your TeX "
+                f"package manager") from None
 
     def _parse_and_cache_line(self, line):
         """

lib/matplotlib/tests/test_dviread.py

@@ -54,9 +54,9 @@ def test_PsfontsMap(monkeypatch):
     entry = fontmap[b'TeXfontC']
     assert entry.psname == b'PSfontC3'
     # Missing font
-    with pytest.raises(KeyError, match='no-such-font'):
+    with pytest.raises(LookupError, match='no-such-font'):
         fontmap[b'no-such-font']
-    with pytest.raises(KeyError, match='%'):
+    with pytest.raises(LookupError, match='%'):
         fontmap[b'%']