Index: Lib/warnings.py
===================================================================
--- Lib/warnings.py	(revision 54927)
+++ Lib/warnings.py	(working copy)
@@ -30,36 +30,26 @@
     if category is None:
         category = UserWarning
     assert issubclass(category, Warning)
+
     # Get context information
     try:
         caller = sys._getframe(stacklevel)
     except ValueError:
-        globals = sys.__dict__
+        # This should only happen if given an inappropriately high
+        # stacklevel.  It can also happen if called from C during
+        # startup/shutdown, when the python stack is empty.
+        module_globals = sys.__dict__
         lineno = 1
+        filename = "<no context>"
     else:
-        globals = caller.f_globals
+        module_globals = caller.f_globals
         lineno = caller.f_lineno
-    if '__name__' in globals:
-        module = globals['__name__']
-    else:
-        module = "<string>"
-    filename = globals.get('__file__')
-    if filename:
-        fnl = filename.lower()
-        if fnl.endswith((".pyc", ".pyo")):
-            filename = filename[:-1]
-    else:
-        if module == "__main__":
-            try:
-                filename = sys.argv[0]
-            except AttributeError:
-                # embedded interpreters don't have sys.argv, see bug #839151
-                filename = '__main__'
-        if not filename:
-            filename = module
-    registry = globals.setdefault("__warningregistry__", {})
+        filename = caller.f_code.co_filename
+
+    module = module_globals.get('__name__', "<string>")
+    registry = module_globals.setdefault("__warningregistry__", {})
     warn_explicit(message, category, filename, lineno, module, registry,
-                  globals)
+                  module_globals)
 
 def warn_explicit(message, category, filename, lineno,
                   module=None, registry=None, module_globals=None):
Index: Lib/test/test_warnings.py
===================================================================
--- Lib/test/test_warnings.py	(revision 54927)
+++ Lib/test/test_warnings.py	(working copy)
@@ -1,5 +1,6 @@
 import warnings
 import os
+import sys
 import unittest
 from test import test_support
 
@@ -77,31 +78,38 @@
             self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
             warning_tests.outer("spam2")
             self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
+            warning_tests.string("spam3")
+            self.assertEqual(w.filename, "<string>")
 
     def test_stacklevel(self):
         # Test stacklevel argument
         # make sure all messages are different, so the warning won't be skipped
         with test_support.catch_warning() as w:
-            warning_tests.inner("spam3", stacklevel=1)
+            warning_tests.inner("spam4", stacklevel=1)
             self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
-            warning_tests.outer("spam4", stacklevel=1)
+            warning_tests.outer("spam5", stacklevel=1)
             self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
+            warning_tests.string("spam6", stacklevel=1)
+            self.assertEqual(w.filename, "<string>")
 
-            warning_tests.inner("spam5", stacklevel=2)
+            warning_tests.inner("spam7", stacklevel=2)
             self.assertEqual(os.path.basename(w.filename), "test_warnings.py")
-            warning_tests.outer("spam6", stacklevel=2)
+            warning_tests.outer("spam8", stacklevel=2)
             self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
+            warning_tests.string("spam9", stacklevel=2)
+            self.assertEqual(os.path.basename(w.filename), "warning_tests.py")
 
-            warning_tests.inner("spam7", stacklevel=9999)
-            self.assertEqual(os.path.basename(w.filename), "sys")
+            warning_tests.inner("spam10", stacklevel=9999)
+            self.assertEqual(w.filename, "<no context>")
 
 
 def test_main(verbose=None):
-    # Obscure hack so that this test passes after reloads or repeated calls
-    # to test_main (regrtest -R).
-    if '__warningregistry__' in globals():
-        del globals()['__warningregistry__']
-    test_support.run_unittest(TestModule)
+    # Entries in the various '__warningregistry__' objects persist
+    # across test runs, causing regrtest -R to fail.
+    # guard_warnings_registry protects against this.
+    with test_support.guard_warnings_registry(sys, warning_tests,
+            sys.modules[__name__]):
+        test_support.run_unittest(TestModule)
 
 if __name__ == "__main__":
     test_main(verbose=True)
Index: Lib/test/warning_tests.py
===================================================================
--- Lib/test/warning_tests.py	(revision 54927)
+++ Lib/test/warning_tests.py	(working copy)
@@ -7,3 +7,6 @@
 
 def inner(message, stacklevel=1):
     warnings.warn(message, stacklevel=stacklevel)
+
+def string(message, stacklevel=1):
+    eval('warnings.warn(message, stacklevel=stacklevel)')
Index: Lib/test/test_support.py
===================================================================
--- Lib/test/test_support.py	(revision 54927)
+++ Lib/test/test_support.py	(working copy)
@@ -282,6 +282,21 @@
     finally:
         warnings.filters = original_filters
 
+@contextlib.contextmanager
+def guard_warnings_registry(*modules):
+    """Guard the warning registries from being permanently changed.
+
+    Must be given one or more modules who's registries are to be protected.
+    """
+    assert modules
+    backups = [getattr(mod, "__warningregistry__", {}).copy()
+        for mod in modules]
+    try:
+        yield
+    finally:
+        for mod, backup in zip(modules, backups):
+            mod.__warningregistry__ = backup
+
 class WarningMessage(object):
     "Holds the result of the latest showwarning() call"
     def __init__(self):