# HG changeset patch
# Parent  40db18240177731ca92fce1717c34e8e7c2e5a67
Issue #18383: Avoid adding duplicate filters when warnings is reloaded

diff -r 40db18240177 Lib/test/test_warnings/__init__.py
--- a/Lib/test/test_warnings/__init__.py	Sun May 22 03:10:11 2016 +0000
+++ b/Lib/test/test_warnings/__init__.py	Sun May 22 04:44:12 2016 +0000
@@ -267,6 +267,53 @@
             self.module.warn_explicit(UserWarning("b"), None, "f.py", 42)
             self.assertEqual(str(w[-1].message), "b")
 
+    def test_filterwarnings_duplicate_filters(self):
+        with original_warnings.catch_warnings(module=self.module):
+            initial_len = len(self.module.filters)
+            self.module.filterwarnings("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1)
+            self.module.filterwarnings("ignore", category=UserWarning)
+            self.module.filterwarnings("error", category=UserWarning)
+            self.assertEqual(
+                len(self.module.filters), initial_len + 2,
+                "filterwarnings inserted duplicate filter"
+            )
+            self.assertEqual(
+                self.module.filters[0][0], "error",
+                "filterwarnings did not promote filter to "
+                "the beginning of list"
+            )
+
+    def test_simplefilter_duplicate_filters(self):
+        with original_warnings.catch_warnings(module=self.module):
+            initial_len = len(self.module.filters)
+            self.module.simplefilter("error", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 1)
+            self.module.simplefilter("ignore", category=UserWarning)
+            self.module.simplefilter("error", category=UserWarning)
+            self.assertEqual(
+                len(self.module.filters), initial_len + 2,
+                "simplefilter inserted duplicate filter"
+            )
+            self.assertEqual(
+                self.module.filters[0][0], "error",
+                "simplefilter did not promote filter to the beginning of list"
+            )
+    def test_append_duplicate(self):
+        with original_warnings.catch_warnings(module=self.module,
+                record=True) as w:
+            initial_len = len(self.module.filters)
+            self.module.simplefilter("ignore")
+            self.module.simplefilter("error", append=True)
+            self.module.simplefilter("ignore", append=True)
+            self.module.warn("test_append_duplicate", category=UserWarning)
+            self.assertEqual(len(self.module.filters), initial_len + 2,
+                "simplefilter inserted duplicate filter"
+            )
+            self.assertEqual(len(w), 0,
+                "appended duplicate changed order of filters"
+            )
+
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings
 
diff -r 40db18240177 Lib/warnings.py
--- a/Lib/warnings.py	Sun May 22 03:10:11 2016 +0000
+++ b/Lib/warnings.py	Sun May 22 04:44:12 2016 +0000
@@ -131,10 +131,18 @@
            "lineno must be an int >= 0"
     item = (action, re.compile(message, re.I), category,
             re.compile(module), lineno)
-    if append:
-        filters.append(item)
+
+    # Remove possible duplicate filters, so new one will be placed
+    # in correct place. If append=True and duplicate exists do nothing.
+    if not append:
+        try:
+            filters.remove(item)
+        except ValueError:
+            pass
+        filters.insert(0, item)
     else:
-        filters.insert(0, item)
+        if item not in filters:
+            filters.append(item)
     _filters_mutated()
 
 def simplefilter(action, category=Warning, lineno=0, append=False):
@@ -152,10 +160,18 @@
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
     item = (action, None, category, None, lineno)
-    if append:
-        filters.append(item)
+
+    # Remove possible duplicate filters, so new one will be placed
+    # in correct place. If append=True and duplicate exists do nothing.
+    if not append:
+        try:
+            filters.remove(item)
+        except ValueError:
+            pass
+        filters.insert(0, item)
     else:
-        filters.insert(0, item)
+        if item not in filters:
+            filters.append(item)
     _filters_mutated()
 
 def resetwarnings():