Issue #27528: Document and test warning messages must match at beginning

1 files changed, 12 insertions, 0 deletions

diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
index a33404e..84a6fb5 100644
--- a/Lib/test/test_warnings/__init__.py
+++ b/Lib/test/test_warnings/__init__.py
@@ -261,6 +261,18 @@ class FilterTests(BaseTest):
             self.assertEqual(str(w[-1].message), text)
             self.assertTrue(w[-1].category is UserWarning)
 
+    def test_message_matching(self):
+        with original_warnings.catch_warnings(record=True,
+                module=self.module) as w:
+            self.module.simplefilter("ignore", UserWarning)
+            self.module.filterwarnings("error", "match", UserWarning)
+            self.assertRaises(UserWarning, self.module.warn, "match")
+            self.assertRaises(UserWarning, self.module.warn, "match prefix")
+            self.module.warn("suffix match")
+            self.assertEqual(w, [])
+            self.module.warn("something completely different")
+            self.assertEqual(w, [])
+
     def test_mutate_filter_list(self):
         class X:
             def match(self, a):