warnings.catch_warnings() now returns a list or None instead of the custom

WarningsRecorder object. This makes the API simpler to use as no special object
must be learned.

Closes issue 3781.
Review by Benjamin Peterson.

Doc/library/warnings.rst

@@ -158,6 +158,67 @@ ImportWarning can also be enabled explicitly in Python code using::
    warnings.simplefilter('default', ImportWarning)
 
 
+.. _warning-suppress:
+
+Temporarily Suppressing Warnings
+--------------------------------
+
+If you are using code that you know will raise a warning, such some deprecated
+function, but do not want to see the warning, then suppress the warning using
+the :class:`catch_warnings` context manager::
+
+    import warnings
+
+    def fxn():
+        warnings.warn("deprecated", DeprecationWarning)
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        fxn()
+
+While within the context manager all warnings will simply be ignored. This
+allows you to use known-deprecated code without having to see the warning while
+not suppressing the warning for other code that might not be aware of its use
+of deprecated code.
+
+
+.. _warning-testing:
+
+Testing Warnings
+----------------
+
+To test warnings raised by code, use the :class:`catch_warnings` context
+manager. With it you can temporarily mutate the warnings filter to facilitate
+your testing. For instance, do the following to capture all raised warnings to
+check::
+
+    import warnings
+
+    def fxn():
+        warnings.warn("deprecated", DeprecationWarning)
+
+    with warnings.catch_warnings(record=True) as w:
+        # Cause all warnings to always be triggered.
+        warnings.simplefilter("always")
+        # Trigger a warning.
+        fxn()
+        # Verify some things
+        assert len(w) == 1
+        assert isinstance(w[-1].category, DeprecationWarning)
+        assert "deprecated" in str(w[-1].message)
+
+One can also cause all warnings to be exceptions by using ``error`` instead of
+``always``. One thing to be aware of is that if a warning has already been
+raised because of a ``once``/``default`` rule, then no matter what filters are
+set the warning will not be seen again unless the warnings registry related to
+the warning has been cleared.
+
+Once the context manager exits, the warnings filter is restored to its state
+when the context was entered. This prevents tests from changing the warnings
+filter in unexpected ways between tests and leading to indeterminate test
+results.
+
+
 .. _warning-functions:
 
 Available Functions
@@ -264,31 +325,22 @@ Available Functions
    and calls to :func:`simplefilter`.
 
 
-Available Classes
------------------
+Available Context Managers
+--------------------------
 
-.. class:: catch_warnings([\*, record=False[, module=None]])
+.. class:: catch_warnings([\*, record=False, module=None])
 
-    A context manager that guards the warnings filter from being permanently
-    mutated. The manager returns an instance of :class:`WarningsRecorder`. The 
-    *record* argument specifies whether warnings that would typically be
-    handled by :func:`showwarning` should instead be recorded by the
-    :class:`WarningsRecorder` instance. This argument is typically set when
-    testing for expected warnings behavior. The *module* argument may be a
-    module object that is to be used instead of the :mod:`warnings` module.
-    This argument should only be set when testing the :mod:`warnings` module 
-    or some similar use-case.
-
-    Typical usage of the context manager is like so::
-
-        def fxn():
-            warn("fxn is deprecated", DeprecationWarning)
-            return "spam spam bacon spam"
+    A context manager that copies and, upon exit, restores the warnings filter.
+    If the *record* argument is False (the default) the context manager returns
+    :class:`None`. If *record* is true, a list is returned that is populated
+    with objects as seen by a custom :func:`showwarning` function (which also
+    suppresses output to ``sys.stdout``). Each object has attributes with the
+    same names as the arguments to :func:`showwarning`.
 
-        # The function 'fxn' is known to raise a DeprecationWarning.
-        with catch_warnings() as w:
-            warnings.filterwarning('ignore', 'fxn is deprecated', DeprecationWarning)
-            fxn()  # DeprecationWarning is temporarily suppressed.
+    The *module* argument takes a module that will be used instead of the
+    module returned when you import :mod:`warnings` whose filter will be
+    protected. This arguments exists primarily for testing the :mod:`warnings`
+    module itself.
 
     .. note::
 
@@ -297,19 +349,3 @@ Available Classes
 
     .. versionadded:: 2.6
 
-
-.. class:: WarningsRecorder()
-
-    A subclass of :class:`list` that stores all warnings passed to
-    :func:`showwarning` when returned by a :class:`catch_warnings` context
-    manager created with its *record* argument set to ``True``. Each recorded
-    warning is represented by an object whose attributes correspond to the
-    arguments to :func:`showwarning`. As a convenience, a
-    :class:`WarningsRecorder` instance has the attributes of the last
-    recorded warning set on the :class:`WarningsRecorder` instance as well.
-
-    .. method:: reset()
-
-        Delete all recorded warnings.
-
-    .. versionadded:: 2.6

Lib/asynchat.py

@@ -50,7 +50,6 @@ import socket
 import asyncore
 from collections import deque
 from sys import py3kwarning
-from test.test_support import catch_warning
 from warnings import filterwarnings, catch_warnings
 
 class async_chat (asyncore.dispatcher):

Lib/bsddb/test/test_early_close.py

@@ -168,9 +168,9 @@ class DBEnvClosedEarlyCrash(unittest.TestCase):
         self.assertEquals(("XXX", "yyy"), c1.first())
         import warnings
         # Not interested in warnings about implicit close.
+        with warnings.catch_warnings():
             warnings.simplefilter("ignore")
             txn.commit()
-        warnings.resetwarnings()
         self.assertRaises(db.DBCursorClosedError, c2.first)
 
     if db.version() > (4,3,0) :

Lib/mimetools.py

@@ -5,7 +5,7 @@ import os
 import sys
 import tempfile
 from warnings import filterwarnings, catch_warnings
-with catch_warnings(record=False):
+with catch_warnings():
     if sys.py3kwarning:
         filterwarnings("ignore", ".*rfc822 has been removed", DeprecationWarning)
     import rfc822

Lib/test/test___all__.py

 import unittest
-from test.test_support import run_unittest, catch_warning
+from test.test_support import run_unittest
 import sys
 import warnings
 
@@ -9,7 +9,7 @@ class AllTest(unittest.TestCase):
 
     def check_all(self, modname):
         names = {}
-        with catch_warning():
+        with warnings.catch_warnings():
             warnings.filterwarnings("ignore", ".* (module|package)",
                                     DeprecationWarning)
             try:

Lib/test/test_exceptions.py

@@ -4,9 +4,9 @@ import os
 import sys
 import unittest
 import pickle, cPickle
+import warnings
 
-from test.test_support import (TESTFN, unlink, run_unittest,
-                                catch_warning, captured_output)
+from test.test_support import TESTFN, unlink, run_unittest, captured_output
 from test.test_pep352 import ignore_message_warning
 
 # XXX This is not really enough, each *operation* should be tested!
@@ -274,7 +274,7 @@ class ExceptionTests(unittest.TestCase):
         except NameError:
             pass
 
-        with catch_warning():
+        with warnings.catch_warnings():
             ignore_message_warning()
             for exc, args, expected in exceptionList:
                 try:

Lib/test/test_hmac.py

@@ -211,7 +211,7 @@ class TestVectorsTestCase(unittest.TestCase):
             def digest(self):
                 return self._x.digest()
 
-        with test_support.catch_warning():
+        with warnings.catch_warnings():
             warnings.simplefilter('error', RuntimeWarning)
             try:
                 hmac.HMAC('a', 'b', digestmod=MockCrazyHash)

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, catch_warning
+from test.test_support import unlink, TESTFN, unload, run_unittest
 
 
 def remove_files(name):
@@ -215,7 +215,7 @@ class ImportTest(unittest.TestCase):
         self.assert_(y is test.test_support, y.__name__)
 
     def test_import_initless_directory_warning(self):
-        with catch_warning():
+        with warnings.catch_warnings():
             # Just a random non-package directory we always expect to be
             # somewhere in sys.path...
             warnings.simplefilter('error', ImportWarning)
@@ -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 catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             check_absolute()
-            self.assert_('foo' in str(w.message))
-            self.assertEqual(w.category, RuntimeWarning)
+            self.assert_('foo' in str(w[-1].message))
+            self.assertEqual(w[-1].category, RuntimeWarning)
         self.assertRaises(SystemError, check_relative)
         # Check relative fails with __package__ and __name__ wrong
         ns = dict(__package__='foo', __name__='notarealpkg.notarealmodule')
-        with catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             check_absolute()
-            self.assert_('foo' in str(w.message))
-            self.assertEqual(w.category, RuntimeWarning)
+            self.assert_('foo' in str(w[-1].message))
+            self.assertEqual(w[-1].category, RuntimeWarning)
         self.assertRaises(SystemError, check_relative)
         # Check both fail with package set to a non-string
         ns = dict(__package__=object())

Lib/test/test_macostools.py

@@ -52,7 +52,7 @@ class TestMacostools(unittest.TestCase):
     def test_touched(self):
         # This really only tests that nothing unforeseen happens.
         import warnings
-        with test_support.catch_warning():
+        with warnings.catch_warnings():
             warnings.filterwarnings('ignore', 'macostools.touched*',
                                     DeprecationWarning)
             macostools.touched(test_support.TESTFN)

Lib/test/test_pep352.py

@@ -2,7 +2,7 @@ import unittest
 import __builtin__
 import exceptions
 import warnings
-from test.test_support import run_unittest, catch_warning
+from test.test_support import run_unittest
 import os
 from platform import system as platform_system
 
@@ -22,7 +22,7 @@ class ExceptionClassTests(unittest.TestCase):
         self.failUnless(issubclass(Exception, object))
 
     def verify_instance_interface(self, ins):
-        with catch_warning():
+        with warnings.catch_warnings():
             ignore_message_warning()
             for attr in ("args", "message", "__str__", "__repr__",
                             "__getitem__"):
@@ -95,7 +95,7 @@ class ExceptionClassTests(unittest.TestCase):
         # Make sure interface works properly when given a single argument
         arg = "spam"
         exc = Exception(arg)
-        with catch_warning():
+        with warnings.catch_warnings():
             ignore_message_warning()
             results = ([len(exc.args), 1], [exc.args[0], arg],
                     [exc.message, arg],
@@ -109,7 +109,7 @@ class ExceptionClassTests(unittest.TestCase):
         arg_count = 3
         args = tuple(range(arg_count))
         exc = Exception(*args)
-        with catch_warning():
+        with warnings.catch_warnings():
             ignore_message_warning()
             results = ([len(exc.args), arg_count], [exc.args, args],
                     [exc.message, ''], [str(exc), str(args)],
@@ -121,7 +121,7 @@ class ExceptionClassTests(unittest.TestCase):
     def test_interface_no_arg(self):
         # Make sure that with no args that interface is correct
         exc = Exception()
-        with catch_warning():
+        with warnings.catch_warnings():
             ignore_message_warning()
             results = ([len(exc.args), 0], [exc.args, tuple()],
                     [exc.message, ''],
@@ -132,7 +132,7 @@ class ExceptionClassTests(unittest.TestCase):
 
     def test_message_deprecation(self):
         # As of Python 2.6, BaseException.message is deprecated.
-        with catch_warning():
+        with warnings.catch_warnings():
             warnings.resetwarnings()
             warnings.filterwarnings('error')
 
@@ -219,7 +219,7 @@ class UsageTests(unittest.TestCase):
 
     def test_catch_string(self):
         # Catching a string should trigger a DeprecationWarning.
-        with catch_warning():
+        with warnings.catch_warnings():
             warnings.resetwarnings()
             warnings.filterwarnings("error")
             str_exc = "spam"

Lib/test/test_random.py

@@ -191,7 +191,7 @@ class WichmannHill_TestBasicOps(TestBasicOps):
 
     def test_bigrand(self):
         # Verify warnings are raised when randrange is too large for random()
-        with test_support.catch_warning():
+        with warnings.catch_warnings():
             warnings.filterwarnings("error", "Underlying random")
             self.assertRaises(UserWarning, self.gen.randrange, 2**60)
 

Lib/test/test_re.py

 import sys
 sys.path = ['.'] + sys.path
 
-from test.test_support import verbose, run_unittest, catch_warning
+from test.test_support import verbose, run_unittest
 import re
 from re import Scanner
 import sys, os, traceback
@@ -447,7 +447,7 @@ class ReTests(unittest.TestCase):
         self.pickle_test(cPickle)
         # old pickles expect the _compile() reconstructor in sre module
         import warnings
-        with catch_warning():
+        with warnings.catch_warnings():
             warnings.filterwarnings("ignore", "The sre module is deprecated",
                                     DeprecationWarning)
             from sre import _compile

Lib/test/test_struct.py

@@ -4,7 +4,7 @@ import struct
 import warnings
 
 from functools import wraps
-from test.test_support import TestFailed, verbose, run_unittest, catch_warning
+from test.test_support import TestFailed, verbose, run_unittest
 
 import sys
 ISBIGENDIAN = sys.byteorder == "big"
@@ -34,7 +34,7 @@ def bigendian_to_native(value):
 def with_warning_restore(func):
     @wraps(func)
     def decorator(*args, **kw):
-        with catch_warning():
+        with warnings.catch_warnings():
             # We need this function to warn every time, so stick an
             # unqualifed 'always' at the head of the filter list
             warnings.simplefilter("always")

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.category, RuntimeWarning)
+        self.assertEqual(w[-1].category, RuntimeWarning)
 
     def test_byte_max(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_BYTE = CHAR_MAX+1
             self.has_warned(w)
 
     def test_byte_min(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_BYTE = CHAR_MIN-1
             self.has_warned(w)
 
     def test_ubyte_max(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_UBYTE = UCHAR_MAX+1
             self.has_warned(w)
 
     def test_short_max(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_SHORT = SHRT_MAX+1
             self.has_warned(w)
 
     def test_short_min(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_SHORT = SHRT_MIN-1
             self.has_warned(w)
 
     def test_ushort_max(self):
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             ts.T_USHORT = USHRT_MAX+1
             self.has_warned(w)
 

Lib/test/test_sundry.py

@@ -8,7 +8,7 @@ import warnings
 
 class TestUntestedModules(unittest.TestCase):
     def test_at_least_import_untested_modules(self):
-        with test_support.catch_warning():
+        with warnings.catch_warnings(record=True):
             import CGIHTTPServer
             import aifc
             import audiodev

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", "catch_warning", "CleanImport",
+           "open_urlresource", "CleanImport",
            "EnvironmentVarGuard", "captured_output",
            "captured_stdout", "TransientResource", "transient_internet",
            "run_with_locale", "set_memlimit", "bigmemtest", "bigaddrspacetest",
@@ -52,7 +52,7 @@ class ResourceDenied(TestSkipped):
 def import_module(name, deprecated=False):
     """Import the module to be tested, raising TestSkipped if it is not
     available."""
-    with catch_warning(record=False):
+    with warnings.catch_warnings():
         if deprecated:
             warnings.filterwarnings("ignore", ".+ (module|package)",
                                     DeprecationWarning)
@@ -381,10 +381,6 @@ def open_urlresource(url):
     return open(fn)
 
 
-def catch_warning(module=warnings, record=True):
-    return warnings.catch_warnings(record=record, module=module)
-
-
 class CleanImport(object):
     """Context manager to force import to return a new module reference.
 

Lib/test/test_symtable.py

@@ -44,7 +44,7 @@ def find_block(block, name):
 
 class SymtableTest(unittest.TestCase):
 
-    with test_support.catch_warning(record=False):
+    with warnings.catch_warnings():
         # Ignore warnings about "from blank import *"
         warnings.simplefilter("ignore", SyntaxWarning)
         top = symtable.symtable(TEST_CODE, "?", "exec")
@@ -60,16 +60,16 @@ class SymtableTest(unittest.TestCase):
         def check(w, msg):
             self.assertEqual(str(w.message), msg)
         sym = self.top.lookup("glob")
-        with test_support.catch_warning() as w:
+        with warnings.catch_warnings(record=True) as w:
             warnings.simplefilter("always", DeprecationWarning)
             self.assertFalse(sym.is_vararg())
-            check(w, "is_vararg() is obsolete and will be removed")
-            w.reset()
+            check(w[-1].message, "is_vararg() is obsolete and will be removed")
             self.assertFalse(sym.is_keywordarg())
-            check(w, "is_keywordarg() is obsolete and will be removed")
-            w.reset()
+            check(w[-1].message,
+                    "is_keywordarg() is obsolete and will be removed")
             self.assertFalse(sym.is_in_tuple())
-            check(w, "is_in_tuple() is obsolete and will be removed")
+            check(w[-1].message,
+                    "is_in_tuple() is obsolete and will be removed")
 
     def test_type(self):
         self.assertEqual(self.top.get_type(), "module")

Lib/test/test_urllib.py

@@ -641,7 +641,7 @@ class Pathname_Tests(unittest.TestCase):
 
 def test_main():
     import warnings
-    with test_support.catch_warning(record=False):
+    with warnings.catch_warnings():
         warnings.filterwarnings('ignore', ".*urllib\.urlopen.*Python 3.0",
                                 DeprecationWarning)
         test_support.run_unittest(

Lib/test/test_urllibnet.py

@@ -182,8 +182,8 @@ class urlretrieveNetworkTests(unittest.TestCase):
 
 def test_main():
     test_support.requires('network')
-    from warnings import filterwarnings
-    with test_support.catch_warning(record=False):
+    from warnings import filterwarnings, catch_warnings
+    with catch_warnings():
         filterwarnings('ignore', '.*urllib\.urlopen.*Python 3.0',
                         DeprecationWarning)
         test_support.run_unittest(URLTimeoutTest,

Lib/test/test_userstring.py

@@ -135,7 +135,7 @@ class MutableStringTest(UserStringTest):
         self.assertEqual(s, "")
 
 def test_main():
-    with test_support.catch_warning(record=False):
+    with warnings.catch_warnings():
         warnings.filterwarnings("ignore", ".*MutableString",
                                 DeprecationWarning)
         test_support.run_unittest(UserStringTest, MutableStringTest)

Lib/warnings.py

@@ -8,7 +8,7 @@ import sys
 import types
 
 __all__ = ["warn", "showwarning", "formatwarning", "filterwarnings",
-           "resetwarnings"]
+           "resetwarnings", "catch_warnings"]
 
 
 def warnpy3k(message, category=None, stacklevel=1):
@@ -304,37 +304,20 @@ class WarningMessage(object):
                                     self.filename, self.lineno, self.line))
 
 
-class WarningsRecorder(list):
-
-    """Record the result of various showwarning() calls."""
-
-    # Explicitly stated arguments so as to not trigger DeprecationWarning
-    # about adding 'line'.
-    def showwarning(self, *args, **kwargs):
-        self.append(WarningMessage(*args, **kwargs))
-
-    def __getattr__(self, attr):
-        """Return attributes from the last caught warning, or raise
-        AttributeError."""
-        try:
-            return getattr(self[-1], attr)
-        except IndexError:
-            raise AttributeError("no recorded warning to read "
-                                    "{0!r} attribute from".format(attr))
-
-
-    def reset(self):
-        del self[:]
-
-
 class catch_warnings(object):
 
-    """Guard the warnings filter from being permanently changed and optionally
-    record the details of any warnings that are issued.
+    """A context manager that copies and restores the warnings filter upon
+    exiting the context.
 
-    Context manager returns an instance of warnings.WarningRecorder which is a
-    list of WarningMessage instances. Attributes on WarningRecorder are
-    redirected to the last created WarningMessage instance.
+    The 'record' argument specifies whether warnings should be captured by a
+    custom implementation of warnings.showwarning() and be appended to a list
+    returned by the context manager. Otherwise None is returned by the context
+    manager. The objects appended to the list are arguments whose attributes
+    mirror the arguments to showwarning().
+
+    The 'module' argument is to specify an alternative module to the module
+    named 'warnings' and imported under that name. This argument is only useful
+    when testing the warnings module itself.
 
     """
 
@@ -346,17 +329,21 @@ class catch_warnings(object):
         keyword-only.
 
         """
-        self._recorder = WarningsRecorder() if record else None
+        self._record = record
         self._module = sys.modules['warnings'] if module is None else module
 
     def __enter__(self):
         self._filters = self._module.filters
         self._module.filters = self._filters[:]
         self._showwarning = self._module.showwarning
-        if self._recorder is not None:
-            self._recorder.reset()  # In case the instance is being reused.
-            self._module.showwarning = self._recorder.showwarning
-        return self._recorder
+        if self._record:
+            log = []
+            def showwarning(*args, **kwargs):
+                log.append(WarningMessage(*args, **kwargs))
+            self._module.showwarning = showwarning
+            return log
+        else:
+            return None
 
     def __exit__(self, *exc_info):
         self._module.filters = self._filters

Misc/NEWS

@@ -60,6 +60,9 @@ C-API
 Library
 -------
 
+- Issue 3781: Clean up the API for warnings.catch_warnings() by having it
+  return a list or None rather than a custom object.
+
 - Issue #1638033: Cookie.Morsel gained the httponly attribute.
 
 - Issue #3535: zipfile couldn't read some zip files larger than 2GB.