Issue #3781: Final cleanup of warnings.catch_warnings and its usage in the test suite. Closes issue w.r.t. 2.6 (R: Brett Cannon)

8 files changed, 200 insertions, 64 deletions

diff --git a/Lib/test/test_import.py b/Lib/test/test_import.py
index 644a473..13e8cc3 100644
--- a/Lib/test/test_import.py
+++ b/Lib/test/test_import.py
@@ -5,7 +5,7 @@ import shutil
 import sys
 import py_compile
 import warnings
-from test.test_support import unlink, TESTFN, unload, run_unittest
+from test.test_support import unlink, TESTFN, unload, run_unittest, check_warnings
 
 
 def remove_files(name):
@@ -279,17 +279,17 @@ class RelativeImport(unittest.TestCase):
         check_relative()
         # Check relative fails with only __package__ wrong
         ns = dict(__package__='foo', __name__='test.notarealmodule')
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             check_absolute()
-            self.assert_('foo' in str(w[-1].message))
-            self.assertEqual(w[-1].category, RuntimeWarning)
+            self.assert_('foo' in str(w.message))
+            self.assertEqual(w.category, RuntimeWarning)
         self.assertRaises(SystemError, check_relative)
         # Check relative fails with __package__ and __name__ wrong
         ns = dict(__package__='foo', __name__='notarealpkg.notarealmodule')
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             check_absolute()
-            self.assert_('foo' in str(w[-1].message))
-            self.assertEqual(w[-1].category, RuntimeWarning)
+            self.assert_('foo' in str(w.message))
+            self.assertEqual(w.category, RuntimeWarning)
         self.assertRaises(SystemError, check_relative)
         # Check both fail with package set to a non-string
         ns = dict(__package__=object())
diff --git a/Lib/test/test_py3kwarn.py b/Lib/test/test_py3kwarn.py
index aa1ecbb..0afa8e7 100644
--- a/Lib/test/test_py3kwarn.py
+++ b/Lib/test/test_py3kwarn.py
@@ -1,6 +1,7 @@
 import unittest
 import sys
-from test.test_support import CleanImport, TestSkipped, run_unittest
+from test.test_support import (check_warnings, CleanImport,
+                               TestSkipped, run_unittest)
 import warnings
 
 from contextlib import nested
@@ -8,15 +9,22 @@ from contextlib import nested
 if not sys.py3kwarning:
     raise TestSkipped('%s must be run with the -3 flag' % __name__)
 
+def reset_module_registry(module):
+    try:
+        registry = module.__warningregistry__
+    except AttributeError:
+        pass
+    else:
+        registry.clear()
 
 class TestPy3KWarnings(unittest.TestCase):
 
     def assertWarning(self, _, warning, expected_message):
-        self.assertEqual(str(warning[-1].message), expected_message)
+        self.assertEqual(str(warning.message), expected_message)
 
     def test_backquote(self):
         expected = 'backquote not supported in 3.x; use repr()'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             exec "`2`" in {}
         self.assertWarning(None, w, expected)
 
@@ -27,55 +35,71 @@ class TestPy3KWarnings(unittest.TestCase):
             exec expr in {'f' : f}
 
         expected = "assignment to True or False is forbidden in 3.x"
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             safe_exec("True = False")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("False = True")
             self.assertWarning(None, w, expected)
+            w.reset()
             try:
                 safe_exec("obj.False = True")
             except NameError: pass
             self.assertWarning(None, w, expected)
+            w.reset()
             try:
                 safe_exec("obj.True = False")
             except NameError: pass
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("def False(): pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("def True(): pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("class False: pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("class True: pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("def f(True=43): pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("def f(False=None): pass")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("f(False=True)")
             self.assertWarning(None, w, expected)
+            w.reset()
             safe_exec("f(True=1)")
             self.assertWarning(None, w, expected)
 
 
     def test_type_inequality_comparisons(self):
         expected = 'type inequality comparisons not supported in 3.x'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(int < str, w, expected)
+            w.reset()
             self.assertWarning(type < object, w, expected)
 
     def test_object_inequality_comparisons(self):
         expected = 'comparing unequal types not supported in 3.x'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(str < [], w, expected)
+            w.reset()
             self.assertWarning(object() < (1, 2), w, expected)
 
     def test_dict_inequality_comparisons(self):
         expected = 'dict inequality comparisons not supported in 3.x'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning({} < {2:3}, w, expected)
+            w.reset()
             self.assertWarning({} <= {}, w, expected)
+            w.reset()
             self.assertWarning({} > {2:3}, w, expected)
+            w.reset()
             self.assertWarning({2:3} >= {}, w, expected)
 
     def test_cell_inequality_comparisons(self):
@@ -86,8 +110,9 @@ class TestPy3KWarnings(unittest.TestCase):
             return g
         cell0, = f(0).func_closure
         cell1, = f(1).func_closure
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(cell0 == cell1, w, expected)
+            w.reset()
             self.assertWarning(cell0 < cell1, w, expected)
 
     def test_code_inequality_comparisons(self):
@@ -96,10 +121,13 @@ class TestPy3KWarnings(unittest.TestCase):
             pass
         def g(x):
             pass
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(f.func_code < g.func_code, w, expected)
+            w.reset()
             self.assertWarning(f.func_code <= g.func_code, w, expected)
+            w.reset()
             self.assertWarning(f.func_code >= g.func_code, w, expected)
+            w.reset()
             self.assertWarning(f.func_code > g.func_code, w, expected)
 
     def test_builtin_function_or_method_comparisons(self):
@@ -107,10 +135,13 @@ class TestPy3KWarnings(unittest.TestCase):
                     'inequality comparisons not supported in 3.x')
         func = eval
         meth = {}.get
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(func < meth, w, expected)
+            w.reset()
             self.assertWarning(func > meth, w, expected)
+            w.reset()
             self.assertWarning(meth <= func, w, expected)
+            w.reset()
             self.assertWarning(meth >= func, w, expected)
 
     def test_sort_cmp_arg(self):
@@ -118,15 +149,18 @@ class TestPy3KWarnings(unittest.TestCase):
         lst = range(5)
         cmp = lambda x,y: -1
 
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(lst.sort(cmp=cmp), w, expected)
+            w.reset()
             self.assertWarning(sorted(lst, cmp=cmp), w, expected)
+            w.reset()
             self.assertWarning(lst.sort(cmp), w, expected)
+            w.reset()
             self.assertWarning(sorted(lst, cmp), w, expected)
 
     def test_sys_exc_clear(self):
         expected = 'sys.exc_clear() not supported in 3.x; use except clauses'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(sys.exc_clear(), w, expected)
 
     def test_methods_members(self):
@@ -135,17 +169,17 @@ class TestPy3KWarnings(unittest.TestCase):
             __methods__ = ['a']
             __members__ = ['b']
         c = C()
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(dir(c), w, expected)
 
     def test_softspace(self):
         expected = 'file.softspace not supported in 3.x'
         with file(__file__) as f:
-            with warnings.catch_warnings(record=True) as w:
+            with check_warnings() as w:
                 self.assertWarning(f.softspace, w, expected)
             def set():
                 f.softspace = 0
-            with warnings.catch_warnings(record=True) as w:
+            with check_warnings() as w:
                 self.assertWarning(set(), w, expected)
 
     def test_slice_methods(self):
@@ -161,59 +195,60 @@ class TestPy3KWarnings(unittest.TestCase):
         expected = "in 3.x, __{0}slice__ has been removed; use __{0}item__"
 
         for obj in (Spam(), Egg()):
-            with warnings.catch_warnings(record=True) as w:
+            with check_warnings() as w:
                 self.assertWarning(obj[1:2], w, expected.format('get'))
+                w.reset()
                 del obj[3:4]
                 self.assertWarning(None, w, expected.format('del'))
+                w.reset()
                 obj[4:5] = "eggs"
                 self.assertWarning(None, w, expected.format('set'))
 
     def test_tuple_parameter_unpacking(self):
         expected = "tuple parameter unpacking has been removed in 3.x"
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             exec "def f((a, b)): pass"
             self.assertWarning(None, w, expected)
 
     def test_buffer(self):
         expected = 'buffer() not supported in 3.x'
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             self.assertWarning(buffer('a'), w, expected)
 
     def test_file_xreadlines(self):
         expected = ("f.xreadlines() not supported in 3.x, "
                     "try 'for line in f' instead")
         with file(__file__) as f:
-            with warnings.catch_warnings(record=True) as w:
+            with check_warnings() as w:
                 self.assertWarning(f.xreadlines(), w, expected)
 
     def test_hash_inheritance(self):
-        with warnings.catch_warnings(record=True) as w:
+        with check_warnings() as w:
             # With object as the base class
             class WarnOnlyCmp(object):
                 def __cmp__(self, other): pass
-            self.assertEqual(len(w), 1)
+            self.assertEqual(len(w.warnings), 1)
             self.assertWarning(None, w,
                  "Overriding __cmp__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class WarnOnlyEq(object):
                 def __eq__(self, other): pass
-            self.assertEqual(len(w), 1)
+            self.assertEqual(len(w.warnings), 1)
             self.assertWarning(None, w,
                  "Overriding __eq__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class WarnCmpAndEq(object):
                 def __cmp__(self, other): pass
                 def __eq__(self, other): pass
-            self.assertEqual(len(w), 2)
-            self.assertWarning(None, w[:1],
+            self.assertEqual(len(w.warnings), 2)
+            self.assertWarning(None, w.warnings[0],
                  "Overriding __cmp__ blocks inheritance of __hash__ in 3.x")
             self.assertWarning(None, w,
                  "Overriding __eq__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class NoWarningOnlyHash(object):
                 def __hash__(self): pass
-            self.assertEqual(len(w), 0)
-            del w[:]
+            self.assertEqual(len(w.warnings), 0)
             # With an intermediate class in the heirarchy
             class DefinesAllThree(object):
                 def __cmp__(self, other): pass
@@ -221,28 +256,28 @@ class TestPy3KWarnings(unittest.TestCase):
                 def __hash__(self): pass
             class WarnOnlyCmp(DefinesAllThree):
                 def __cmp__(self, other): pass
-            self.assertEqual(len(w), 1)
+            self.assertEqual(len(w.warnings), 1)
             self.assertWarning(None, w,
                  "Overriding __cmp__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class WarnOnlyEq(DefinesAllThree):
                 def __eq__(self, other): pass
-            self.assertEqual(len(w), 1)
+            self.assertEqual(len(w.warnings), 1)
             self.assertWarning(None, w,
                  "Overriding __eq__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class WarnCmpAndEq(DefinesAllThree):
                 def __cmp__(self, other): pass
                 def __eq__(self, other): pass
-            self.assertEqual(len(w), 2)
-            self.assertWarning(None, w[:1],
+            self.assertEqual(len(w.warnings), 2)
+            self.assertWarning(None, w.warnings[0],
                  "Overriding __cmp__ blocks inheritance of __hash__ in 3.x")
             self.assertWarning(None, w,
                  "Overriding __eq__ blocks inheritance of __hash__ in 3.x")
-            del w[:]
+            w.reset()
             class NoWarningOnlyHash(DefinesAllThree):
                 def __hash__(self): pass
-            self.assertEqual(len(w), 0)
+            self.assertEqual(len(w.warnings), 0)
 
 
 class TestStdlibRemovals(unittest.TestCase):
@@ -283,6 +318,9 @@ class TestStdlibRemovals(unittest.TestCase):
         """Make sure the specified module, when imported, raises a
         DeprecationWarning and specifies itself in the message."""
         with nested(CleanImport(module_name), warnings.catch_warnings()):
+            # XXX: This is not quite enough for extension modules - those
+            # won't rerun their init code even with CleanImport.
+            # You can see this easily by running the whole test suite with -3
             warnings.filterwarnings("error", ".+ removed",
                                     DeprecationWarning, __name__)
             try:
@@ -320,12 +358,15 @@ class TestStdlibRemovals(unittest.TestCase):
         def dumbo(where, names, args): pass
         for path_mod in ("ntpath", "macpath", "os2emxpath", "posixpath"):
             mod = __import__(path_mod)
-            with warnings.catch_warnings(record=True) as w:
+            reset_module_registry(mod)
+            with check_warnings() as w:
                 mod.walk("crashers", dumbo, None)
-            self.assertEquals(str(w[-1].message), msg)
+            self.assertEquals(str(w.message), msg)
 
     def test_commands_members(self):
         import commands
+        # commands module tests may have already triggered this warning
+        reset_module_registry(commands)
         members = {"mk2arg" : 2, "mkarg" : 1, "getstatus" : 1}
         for name, arg_count in members.items():
             with warnings.catch_warnings():
@@ -335,6 +376,8 @@ class TestStdlibRemovals(unittest.TestCase):
 
     def test_reduce_move(self):
         from operator import add
+        # reduce tests may have already triggered this warning
+        reset_module_registry(unittest)
         with warnings.catch_warnings():
             warnings.filterwarnings("error", "reduce")
             self.assertRaises(DeprecationWarning, reduce, add, range(10))
@@ -342,6 +385,8 @@ class TestStdlibRemovals(unittest.TestCase):
     def test_mutablestring_removal(self):
         # UserString.MutableString has been removed in 3.0.
         import UserString
+        # UserString tests may have already triggered this warning
+        reset_module_registry(UserString)
         with warnings.catch_warnings():
             warnings.filterwarnings("error", ".*MutableString",
                                     DeprecationWarning)
@@ -349,7 +394,7 @@ class TestStdlibRemovals(unittest.TestCase):
 
 
 def test_main():
-    with warnings.catch_warnings():
+    with check_warnings():
         warnings.simplefilter("always")
         run_unittest(TestPy3KWarnings,
                      TestStdlibRemovals)
diff --git a/Lib/test/test_structmembers.py b/Lib/test/test_structmembers.py
index e0e7e56..c196cc5 100644
--- a/Lib/test/test_structmembers.py
+++ b/Lib/test/test_structmembers.py
@@ -66,35 +66,35 @@ class ReadWriteTests(unittest.TestCase):
 
 class TestWarnings(unittest.TestCase):
     def has_warned(self, w):
-        self.assertEqual(w[-1].category, RuntimeWarning)
+        self.assertEqual(w.category, RuntimeWarning)
 
     def test_byte_max(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_BYTE = CHAR_MAX+1
             self.has_warned(w)
 
     def test_byte_min(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_BYTE = CHAR_MIN-1
             self.has_warned(w)
 
     def test_ubyte_max(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_UBYTE = UCHAR_MAX+1
             self.has_warned(w)
 
     def test_short_max(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_SHORT = SHRT_MAX+1
             self.has_warned(w)
 
     def test_short_min(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_SHORT = SHRT_MIN-1
             self.has_warned(w)
 
     def test_ushort_max(self):
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             ts.T_USHORT = USHRT_MAX+1
             self.has_warned(w)
 
diff --git a/Lib/test/test_sundry.py b/Lib/test/test_sundry.py
index 49ec12c..e7499ae 100644
--- a/Lib/test/test_sundry.py
+++ b/Lib/test/test_sundry.py
@@ -8,7 +8,7 @@ import warnings
 
 class TestUntestedModules(unittest.TestCase):
     def test_at_least_import_untested_modules(self):
-        with warnings.catch_warnings(record=True):
+        with warnings.catch_warnings():
             import CGIHTTPServer
             import aifc
             import audiodev
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index 0bf22cf..abaf117 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -18,7 +18,7 @@ __all__ = ["Error", "TestFailed", "TestSkipped", "ResourceDenied", "import_modul
            "is_resource_enabled", "requires", "find_unused_port", "bind_port",
            "fcmp", "have_unicode", "is_jython", "TESTFN", "HOST", "FUZZ",
            "findfile", "verify", "vereq", "sortdict", "check_syntax_error",
-           "open_urlresource", "CleanImport",
+           "open_urlresource", "check_warnings", "CleanImport",
            "EnvironmentVarGuard", "captured_output",
            "captured_stdout", "TransientResource", "transient_internet",
            "run_with_locale", "set_memlimit", "bigmemtest", "bigaddrspacetest",
@@ -381,6 +381,29 @@ def open_urlresource(url):
     return open(fn)
 
 
+class WarningsRecorder(object):
+    """Convenience wrapper for the warnings list returned on
+       entry to the warnings.catch_warnings() context manager.
+    """
+    def __init__(self, warnings_list):
+        self.warnings = warnings_list
+
+    def __getattr__(self, attr):
+        if self.warnings:
+            return getattr(self.warnings[-1], attr)
+        elif attr in warnings.WarningMessage._WARNING_DETAILS:
+            return None
+        raise AttributeError("%r has no attribute %r" % (self, attr))
+
+    def reset(self):
+        del self.warnings[:]
+
+@contextlib.contextmanager
+def check_warnings():
+    with warnings.catch_warnings(record=True) as w:
+        yield WarningsRecorder(w)
+
+
 class CleanImport(object):
     """Context manager to force import to return a new module reference.
 
diff --git a/Lib/test/test_symtable.py b/Lib/test/test_symtable.py
index b20f2b4..f1fa18a 100644
--- a/Lib/test/test_symtable.py
+++ b/Lib/test/test_symtable.py
@@ -60,16 +60,16 @@ class SymtableTest(unittest.TestCase):
         def check(w, msg):
             self.assertEqual(str(w.message), msg)
         sym = self.top.lookup("glob")
-        with warnings.catch_warnings(record=True) as w:
+        with test_support.check_warnings() as w:
             warnings.simplefilter("always", DeprecationWarning)
             self.assertFalse(sym.is_vararg())
-            check(w[-1].message, "is_vararg() is obsolete and will be removed")
+            check(w, "is_vararg() is obsolete and will be removed")
+            w.reset()
             self.assertFalse(sym.is_keywordarg())
-            check(w[-1].message,
-                    "is_keywordarg() is obsolete and will be removed")
+            check(w, "is_keywordarg() is obsolete and will be removed")
+            w.reset()
             self.assertFalse(sym.is_in_tuple())
-            check(w[-1].message,
-                    "is_in_tuple() is obsolete and will be removed")
+            check(w, "is_in_tuple() is obsolete and will be removed")
 
     def test_type(self):
         self.assertEqual(self.top.get_type(), "module")
diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index 388b5e9..b37cdf7 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -517,10 +517,12 @@ class CatchWarningTests(BaseTest):
         wmod = self.module
         orig_filters = wmod.filters
         orig_showwarning = wmod.showwarning
-        with wmod.catch_warnings(record=True, module=wmod):
+        # Ensure both showwarning and filters are restored when recording
+        with wmod.catch_warnings(module=wmod, record=True):
             wmod.filters = wmod.showwarning = object()
         self.assert_(wmod.filters is orig_filters)
         self.assert_(wmod.showwarning is orig_showwarning)
+        # Same test, but with recording disabled
         with wmod.catch_warnings(module=wmod, record=False):
             wmod.filters = wmod.showwarning = object()
         self.assert_(wmod.filters is orig_filters)
@@ -528,9 +530,10 @@ class CatchWarningTests(BaseTest):
 
     def test_catch_warnings_recording(self):
         wmod = self.module
+        # Ensure warnings are recorded when requested
         with wmod.catch_warnings(module=wmod, record=True) as w:
             self.assertEqual(w, [])
-            self.assertRaises(AttributeError, getattr, w, 'message')
+            self.assert_(type(w) is list)
             wmod.simplefilter("always")
             wmod.warn("foo")
             self.assertEqual(str(w[-1].message), "foo")
@@ -540,11 +543,61 @@ class CatchWarningTests(BaseTest):
             self.assertEqual(str(w[1].message), "bar")
             del w[:]
             self.assertEqual(w, [])
+        # Ensure warnings are not recorded when not requested
         orig_showwarning = wmod.showwarning
         with wmod.catch_warnings(module=wmod, record=False) as w:
             self.assert_(w is None)
             self.assert_(wmod.showwarning is orig_showwarning)
 
+    def test_catch_warnings_reentry_guard(self):
+        wmod = self.module
+        # Ensure catch_warnings is protected against incorrect usage
+        x = wmod.catch_warnings(module=wmod, record=True)
+        self.assertRaises(RuntimeError, x.__exit__)
+        with x:
+            self.assertRaises(RuntimeError, x.__enter__)
+        # Same test, but with recording disabled
+        x = wmod.catch_warnings(module=wmod, record=False)
+        self.assertRaises(RuntimeError, x.__exit__)
+        with x:
+            self.assertRaises(RuntimeError, x.__enter__)
+
+    def test_catch_warnings_defaults(self):
+        wmod = self.module
+        orig_filters = wmod.filters
+        orig_showwarning = wmod.showwarning
+        # Ensure default behaviour is not to record warnings
+        with wmod.catch_warnings(module=wmod) as w:
+            self.assert_(w is None)
+            self.assert_(wmod.showwarning is orig_showwarning)
+            self.assert_(wmod.filters is not orig_filters)
+        self.assert_(wmod.filters is orig_filters)
+        if wmod is sys.modules['warnings']:
+            # Ensure the default module is this one
+            with wmod.catch_warnings() as w:
+                self.assert_(w is None)
+                self.assert_(wmod.showwarning is orig_showwarning)
+                self.assert_(wmod.filters is not orig_filters)
+            self.assert_(wmod.filters is orig_filters)
+
+    def test_check_warnings(self):
+        # Explicit tests for the test_support convenience wrapper
+        wmod = self.module
+        if wmod is sys.modules['warnings']:
+            with test_support.check_warnings() as w:
+                self.assertEqual(w.warnings, [])
+                wmod.simplefilter("always")
+                wmod.warn("foo")
+                self.assertEqual(str(w.message), "foo")
+                wmod.warn("bar")
+                self.assertEqual(str(w.message), "bar")
+                self.assertEqual(str(w.warnings[0].message), "foo")
+                self.assertEqual(str(w.warnings[1].message), "bar")
+                w.reset()
+                self.assertEqual(w.warnings, [])
+
+
+
 class CCatchWarningTests(CatchWarningTests):
     module = c_warnings
 
diff --git a/Lib/warnings.py b/Lib/warnings.py
index 04e7b58..59011ca 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -331,8 +331,21 @@ class catch_warnings(object):
         """
         self._record = record
         self._module = sys.modules['warnings'] if module is None else module
+        self._entered = False
+
+    def __repr__(self):
+        args = []
+        if self._record:
+            args.append("record=True")
+        if self._module is not sys.modules['warnings']:
+            args.append("module=%r" % self._module)
+        name = type(self).__name__
+        return "%s(%s)" % (name, ", ".join(args))
 
     def __enter__(self):
+        if self._entered:
+            raise RuntimeError("Cannot enter %r twice" % self)
+        self._entered = True
         self._filters = self._module.filters
         self._module.filters = self._filters[:]
         self._showwarning = self._module.showwarning
@@ -346,6 +359,8 @@ class catch_warnings(object):
             return None
 
     def __exit__(self, *exc_info):
+        if not self._entered:
+            raise RuntimeError("Cannot exit %r without entering first" % self)
         self._module.filters = self._filters
         self._module.showwarning = self._showwarning