I'm only backporting the tests here.

Merged revisions 86395 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r86395 | antoine.pitrou | 2010-11-10 14:55:25 +0100 (mer., 10 nov. 2010) | 4 lines

  Issue #10372: Import the warnings module only after the IO library is
  initialized, so as to avoid bootstrap issues with the '-W' option.
........

Lib/test/script_helper.py

@@ -12,6 +12,45 @@ import shutil
 import zipfile
 
 # Executing the interpreter in a subprocess
+def _assert_python(expected_success, *args, **env_vars):
+    cmd_line = [sys.executable]
+    if not env_vars:
+        cmd_line.append('-E')
+    cmd_line.extend(args)
+    # Need to preserve the original environment, for in-place testing of
+    # shared library builds.
+    env = os.environ.copy()
+    env.update(env_vars)
+    p = subprocess.Popen(cmd_line, stdin=subprocess.PIPE,
+                         stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                         env=env)
+    try:
+        out, err = p.communicate()
+    finally:
+        subprocess._cleanup()
+        p.stdout.close()
+        p.stderr.close()
+    rc = p.returncode
+    if (rc and expected_success) or (not rc and not expected_success):
+        raise AssertionError(
+            "Process return code is %d, "
+            "stderr follows:\n%s" % (rc, err.decode('ascii', 'ignore')))
+    return rc, out, err
+
+def assert_python_ok(*args, **env_vars):
+    """
+    Assert that running the interpreter with `args` and optional environment
+    variables `env_vars` is ok and return a (return code, stdout, stderr) tuple.
+    """
+    return _assert_python(True, *args, **env_vars)
+
+def assert_python_failure(*args, **env_vars):
+    """
+    Assert that running the interpreter with `args` and optional environment
+    variables `env_vars` fails and return a (return code, stdout, stderr) tuple.
+    """
+    return _assert_python(False, *args, **env_vars)
+
 def python_exit_code(*args):
     cmd_line = [sys.executable, '-E']
     cmd_line.extend(args)

Lib/test/test_warnings.py

@@ -6,6 +6,7 @@ import sys
 import unittest
 import subprocess
 from test import test_support
+from test.script_helper import assert_python_ok
 
 import warning_tests
 
@@ -381,6 +382,22 @@ class WCmdLineTests(unittest.TestCase):
             self.module._setoption('error::Warning::0')
             self.assertRaises(UserWarning, self.module.warn, 'convert to error')
 
+    def test_improper_option(self):
+        # Same as above, but check that the message is printed out when
+        # the interpreter is executed. This also checks that options are
+        # actually parsed at all.
+        rc, out, err = assert_python_ok("-Wxxx", "-c", "pass")
+        self.assertIn(b"Invalid -W option ignored: invalid action: 'xxx'", err)
+
+    def test_warnings_bootstrap(self):
+        # Check that the warnings module does get loaded when -W<some option>
+        # is used (see issue #10372 for an example of silent bootstrap failure).
+        rc, out, err = assert_python_ok("-Wi", "-c",
+            "import sys; sys.modules['warnings'].warn('foo', RuntimeWarning)")
+        # '-Wi' was observed
+        self.assertFalse(out.strip())
+        self.assertNotIn(b'RuntimeWarning', err)
+
 class CWCmdLineTests(BaseTest, WCmdLineTests):
     module = c_warnings