Merge pull request matplotlib#25470 from tacaswell/fix/donot_cache_exceptions

FIX: do not cache exceptions

Author: timhoffm

lib/matplotlib/_api/__init__.py

@@ -385,4 +385,6 @@ def warn_external(message, category=None):
                         frame.f_globals.get("__name__", "")):
             break
         frame = frame.f_back
+    # premetively break reference cycle between locals and the frame
+    del frame
     warnings.warn(message, category, stacklevel)

lib/matplotlib/cbook.py

@@ -67,6 +67,8 @@ def _get_running_interactive_framework():
                 if frame.f_code in codes:
                     return "tk"
                 frame = frame.f_back
+        # premetively break reference cycle between locals and the frame
+        del frame
     macosx = sys.modules.get("matplotlib.backends._macosx")
     if macosx and macosx.event_loop_is_running():
         return "macosx"

lib/matplotlib/font_manager.py

@@ -26,6 +26,7 @@
 #   - 'light' is an invalid weight value, remove it.
 
 from base64 import b64encode
+from collections import namedtuple
 import copy
 import dataclasses
 from functools import lru_cache
@@ -128,6 +129,7 @@
     'sans',
 }
 
+_ExceptionProxy = namedtuple('_ExceptionProxy', ['klass', 'message'])
 
 # OS Font paths
 try:
@@ -1288,8 +1290,8 @@ def findfont(self, prop, fontext='ttf', directory=None,
         ret = self._findfont_cached(
             prop, fontext, directory, fallback_to_default, rebuild_if_missing,
             rc_params)
-        if isinstance(ret, Exception):
-            raise ret
+        if isinstance(ret, _ExceptionProxy):
+            raise ret.klass(ret.message)
         return ret
 
     def get_font_names(self):
@@ -1440,10 +1442,12 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                                      fallback_to_default=False)
             else:
                 # This return instead of raise is intentional, as we wish to
-                # cache the resulting exception, which will not occur if it was
+                # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return ValueError(f"Failed to find font {prop}, and fallback "
-                                  f"to the default font was disabled")
+                return _ExceptionProxy(
+                    ValueError,
+                    f"Failed to find font {prop}, and fallback to the default font was disabled"
+                )
         else:
             _log.debug('findfont: Matching %s to %s (%r) with score of %f.',
                        prop, best_font.name, best_font.fname, best_score)
@@ -1463,9 +1467,9 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                     prop, fontext, directory, rebuild_if_missing=False)
             else:
                 # This return instead of raise is intentional, as we wish to
-                # cache the resulting exception, which will not occur if it was
+                # cache that it was not found, which will not occur if it was
                 # actually raised.
-                return ValueError("No valid font could be found")
+                return _ExceptionProxy(ValueError, "No valid font could be found")
 
         return _cached_realpath(result)
 

lib/matplotlib/tests/test_font_manager.py

@@ -1,4 +1,5 @@
 from io import BytesIO, StringIO
+import gc
 import multiprocessing
 import os
 from pathlib import Path
@@ -16,7 +17,7 @@
     json_dump, json_load, get_font, is_opentype_cff_font,
     MSUserFontDirectories, _get_fontconfig_fonts, ft2font,
     ttfFontProperty, cbook)
-from matplotlib import pyplot as plt, rc_context
+from matplotlib import pyplot as plt, rc_context, figure as mfigure
 
 has_fclist = shutil.which('fc-list') is not None
 
@@ -324,3 +325,25 @@ def test_get_font_names():
     assert set(available_fonts) == set(mpl_font_names)
     assert len(available_fonts) == len(mpl_font_names)
     assert available_fonts == mpl_font_names
+
+
+def test_donot_cache_tracebacks():
+
+    class SomeObject:
+        pass
+
+    def inner():
+        x = SomeObject()
+        fig = mfigure.Figure()
+        ax = fig.subplots()
+        fig.text(.5, .5, 'aardvark', family='doesnotexist')
+        with BytesIO() as out:
+            with warnings.catch_warnings():
+                warnings.filterwarnings('ignore')
+                fig.savefig(out, format='png')
+
+    inner()
+
+    for obj in gc.get_objects():
+        if isinstance(obj, SomeObject):
+            pytest.fail("object from inner stack still alive")