From 221945056da4540506ff2303c07324e60b234e57 Mon Sep 17 00:00:00 2001
From: Martin Panter <vadmium+py@gmail.com>
Date: Tue, 19 Jul 2016 02:26:38 +0000
Subject: [PATCH] Issue #27528: Document and test warning messages must match
 at beginning

---
 Doc/library/warnings.rst           |  7 ++++---
 Lib/test/test_warnings/__init__.py | 12 ++++++++++++
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index 5212131764d..37f6874a02b 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -141,14 +141,15 @@ the disposition of the match.  Each entry is a tuple of the form (*action*,
   |               | warnings, regardless of location             |
   +---------------+----------------------------------------------+
 
-* *message* is a string containing a regular expression that the warning message
-  must match (the match is compiled to always be case-insensitive).
+* *message* is a string containing a regular expression that the start of
+  the warning message must match.  The expression is compiled to always be
+  case-insensitive.
 
 * *category* is a class (a subclass of :exc:`Warning`) of which the warning
   category must be a subclass in order to match.
 
 * *module* is a string containing a regular expression that the module name must
-  match (the match is compiled to be case-sensitive).
+  match.  The expression is compiled to be case-sensitive.
 
 * *lineno* is an integer that the line number where the warning occurred must
   match, or ``0`` to match all line numbers.
diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
index a33404ef853..84a6fb58426 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):