TST: Add unit test to catch recurrences of matplotlib#20822, matplotlib#20855

Author: lpsinger

lib/matplotlib/tests/test_getattr.py

@@ -0,0 +1,28 @@
+from importlib import import_module
+from pkgutil import walk_packages
+
+import matplotlib
+import pytest
+
+# Get the names of all matplotlib submodules, except for the unit tests.
+module_names = [m.name for m in walk_packages(path=matplotlib.__path__,
+                                              prefix=f'{matplotlib.__name__}.')
+                if not m.name.startswith(__package__)]
+
+
+@pytest.mark.parametrize('module_name', module_names)
+@pytest.mark.filterwarnings('ignore::DeprecationWarning')
+def test_getattr(module_name):
+    """
+    Test that __getattr__ methods raise AttributeError for unknown keys.
+    See #20822, #20855.
+    """
+    try:
+        module = import_module(module_name)
+    except (ImportError, RuntimeError) as e:
+        # Skip modules that cannot be imported due to missing dependencies
+        pytest.skip(f'Cannot import {module_name} due to {e}')
+
+    key = 'THIS_SYMBOL_SHOULD_NOT_EXIST'
+    if hasattr(module, key):
+        delattr(module, key)