Index: Python/_warnings.c
===================================================================
--- Python/_warnings.c	(revision 65970)
+++ Python/_warnings.c	(working copy)
@@ -258,6 +258,8 @@
     /* Print "  source_line\n" */
     if (sourceline) {
         char *source_line_str = _PyUnicode_AsString(sourceline);
+	if (source_line_str == NULL)
+		return;
         while (*source_line_str == ' ' || *source_line_str == '\t' ||
                 *source_line_str == '\014')
             source_line_str++;
@@ -266,8 +268,9 @@
         PyFile_WriteString("\n", f_stderr);
     }
     else
-        _Py_DisplaySourceLine(f_stderr, _PyUnicode_AsString(filename),
-                              lineno, 2);
+        if (_Py_DisplaySourceLine(f_stderr, _PyUnicode_AsString(filename),
+                              lineno, 2) < 0)
+		return;
     PyErr_Clear();
 }
 
@@ -366,8 +369,11 @@
             PyObject *to_str = PyObject_Str(item);
             const char *err_str = "???";
 
-            if (to_str != NULL)
+            if (to_str != NULL) {
                 err_str = _PyUnicode_AsString(to_str);
+		if (err_str == NULL)
+			goto cleanup;
+	    }
             PyErr_Format(PyExc_RuntimeError,
                         "Unrecognized action (%s) in warnings.filters:\n %s",
                         action, err_str);
@@ -388,9 +394,15 @@
         else {
             const char *msg = "functions overriding warnings.showwarning() "
                                 "must support the 'line' argument";
-            const char *text_char = _PyUnicode_AsString(text);
+            const char *text_char = NULL;
+	    
+	    if (PyUnicode_Check(text)) {
+		    text_char = _PyUnicode_AsString(text);
+		    if (text_char == NULL)
+			    goto cleanup;
+	    }
 
-            if (strcmp(msg, text_char) == 0) {
+            if (text_char && strcmp(msg, text_char) == 0) {
                 /* Prevent infinite recursion by using built-in implementation
                    of showwarning(). */
                 show_warning(filename, lineno, text, category, sourceline);
@@ -524,7 +536,9 @@
     }
     else {
         const char *module_str = _PyUnicode_AsString(*module);
-        if (module_str && strcmp(module_str, "__main__") == 0) {
+	if (module_str == NULL)
+		goto handle_error;
+        if (strcmp(module_str, "__main__") == 0) {
             PyObject *argv = PySys_GetObject("argv");
             if (argv != NULL && PyList_Size(argv) > 0) {
                 int is_true;
Index: Lib/test/test_warnings.py
===================================================================
--- Lib/test/test_warnings.py	(revision 65970)
+++ Lib/test/test_warnings.py	(working copy)
@@ -203,6 +203,16 @@
                 self.assertEqual(str(w.message), text)
                 self.assert_(w.category is UserWarning)
 
+    # Issue 3639
+    def test_warn_nonstandard_types(self):
+        # warn() should handle non-standard types without issue.
+        for ob in (Warning, None, 42):
+            with support.catch_warning(self.module) as w:
+                self.module.warn(ob)
+                # Don't directly compare objects since
+                # ``Warning() != Warning()``.
+                self.assertEquals(str(w.message), str(UserWarning(ob)))
+
     def test_filename(self):
         with warnings_state(self.module):
             with support.catch_warning(self.module) as w:
@@ -317,7 +327,6 @@
                             None, Warning, None, 1, registry=42)
 
 
-
 class CWarnTests(BaseTest, WarnTests):
     module = c_warnings