warnings.warn_explicit() did not have the proper TypeErrors in place to prevent

bus errors or SystemError being raised. As a side effect of fixing this, a bad
DECREF that could be triggered when 'message' and 'category' were both None was
fixed.

Closes issue 3211. Thanks JP Calderone for the bug report.

Lib/test/test_warnings.py

@@ -301,6 +301,21 @@ class WarnTests(unittest.TestCase):
             warning_tests.__name__ = module_name
             sys.argv = argv
 
+    def test_warn_explicit_type_errors(self):
+        # warn_explicit() shoud error out gracefully if it is given objects
+        # of the wrong types.
+        # lineno is expected to be an integer.
+        self.assertRaises(TypeError, self.module.warn_explicit,
+                            None, UserWarning, None, None)
+        # Either 'message' needs to be an instance of Warning or 'category'
+        # needs to be a subclass.
+        self.assertRaises(TypeError, self.module.warn_explicit,
+                            None, None, None, 1)
+        # 'registry' must be a dict or None.
+        self.assertRaises((TypeError, AttributeError),
+                            self.module.warn_explicit,
+                            None, Warning, None, 1, registry=42)
+
 
 
 class CWarnTests(BaseTest, WarnTests):

Lib/warnings.py

@@ -202,6 +202,7 @@ def warn(message, category=None, stacklevel=1):
 
 def warn_explicit(message, category, filename, lineno,
                   module=None, registry=None, module_globals=None):
+    lineno = int(lineno)
     if module is None:
         module = filename or "<unknown>"
         if module[-3:].lower() == ".py":