python
diff --git a/‎Lib/test/test_warnings/__init__.py‎
Lines changed: 34 additions & 1 deletion b/‎Lib/test/test_warnings/__init__.py‎
Lines changed: 34 additions & 1 deletion
diff --git a/‎Lib/warnings.py‎
Lines changed: 3 additions & 1 deletion b/‎Lib/warnings.py‎
Lines changed: 3 additions & 1 deletion
diff --git a/‎Python/_warnings.c‎
Lines changed: 33 additions & 7 deletions b/‎Python/_warnings.c‎
Lines changed: 33 additions & 7 deletions
@@ -16,6 +16,8 @@
 py_warnings = support.import_fresh_module('warnings', blocked=['_warnings'])
 c_warnings = support.import_fresh_module('warnings', fresh=['_warnings'])
 
+Py_DEBUG = hasattr(sys, 'gettotalrefcount')
+
 @contextmanager
 def warnings_state(module):
     """Use a specific warnings implementation in warning_tests."""
@@ -320,6 +322,7 @@ def test_simplefilter_duplicate_filters(self):
                 self.module.filters[0][0], "error",
                 "simplefilter did not promote filter to the beginning of list"
             )
+
     def test_append_duplicate(self):
         with original_warnings.catch_warnings(module=self.module,
                 record=True) as w:
@@ -1143,6 +1146,36 @@ def test_conflicting_envvar_and_command_line(self):
              b"  File \"<string>\", line 1, in <module>",
              b"DeprecationWarning: Message"])
 
+    def test_default_filter_configuration(self):
+        pure_python_api = self.module is py_warnings
+        if Py_DEBUG:
+            expected_default_filters = []
+        else:
+            if pure_python_api:
+                main_module_filter = re.compile("__main__")
+            else:
+                main_module_filter = "__main__"
+            expected_default_filters = [
+                ('default', None, DeprecationWarning, main_module_filter, 0),
+                ('ignore', None, DeprecationWarning, None, 0),
+                ('ignore', None, PendingDeprecationWarning, None, 0),
+                ('ignore', None, ImportWarning, None, 0),
+                ('ignore', None, ResourceWarning, None, 0),
+            ]
+        expected_output = "".join(str(f) + "\n" for f in expected_default_filters)
+
+        if pure_python_api:
+            # Disable the warnings acceleration module in the subprocess
+            code = "import sys; sys.modules['_warnings'] = None; "
+        else:
+            code = ""
+        code += "import warnings; [print(f) for f in warnings.filters]"
+
+        rc, stdout, stderr = assert_python_ok("-c", code, __isolated=True)
+        self.maxDiff = None
+        self.assertEqual(stdout.decode(), expected_output)
+
+
     @unittest.skipUnless(sys.getfilesystemencoding() != 'ascii',
                          'requires non-ascii filesystemencoding')
     def test_nonascii(self):
@@ -1192,7 +1225,7 @@ def __del__(self):
         rc, out, err = assert_python_ok("-c", code)
         # note: "__main__" filename is not correct, it should be the name
         # of the script
-        self.assertEqual(err, b'__main__:7: UserWarning: test')
+        self.assertEqual(err.decode(), '__main__:7: UserWarning: test')
 
     def test_late_resource_warning(self):
         # Issue #21925: Emitting a ResourceWarning late during the Python
 
@@ -519,8 +519,10 @@ def _filters_mutated():
 # Module initialization
 _processoptions(sys.warnoptions)
 if not _warnings_defaults:
-    # Several warning categories are ignored by default in Py_DEBUG builds
+    # Several warning categories are ignored by default in regular builds
     if not hasattr(sys, 'gettotalrefcount'):
+        filterwarnings("default", category=DeprecationWarning,
+                       module="__main__", append=1)
         simplefilter("ignore", category=DeprecationWarning, append=1)
         simplefilter("ignore", category=PendingDeprecationWarning, append=1)
         simplefilter("ignore", category=ImportWarning, append=1)
 
@@ -12,6 +12,7 @@ MODULE_NAME " provides basic warning filtering support.\n"
 _Py_IDENTIFIER(argv);
 _Py_IDENTIFIER(stderr);
 #ifndef Py_DEBUG
+_Py_IDENTIFIER(default);
 _Py_IDENTIFIER(ignore);
 #endif
 
@@ -22,8 +23,20 @@ check_matched(PyObject *obj, PyObject *arg)
     _Py_IDENTIFIER(match);
     int rc;
 
+    /* A 'None' filter always matches */
     if (obj == Py_None)
         return 1;
+
+    /* An internal plain text default filter must match exactly */
+    if (PyUnicode_CheckExact(obj)) {
+        int cmp_result = PyUnicode_Compare(obj, arg);
+        if (cmp_result == -1 && PyErr_Occurred()) {
+            return -1;
+        }
+        return !cmp_result;
+    }
+
+    /* Otherwise assume a regex filter and call its match() method */
     result = _PyObject_CallMethodIdObjArgs(obj, &PyId_match, arg, NULL);
     if (result == NULL)
         return -1;
@@ -1158,16 +1171,27 @@ static PyMethodDef warnings_functions[] = {
 
 #ifndef Py_DEBUG
 static PyObject *
-create_filter(PyObject *category, _Py_Identifier *id)
+create_filter(PyObject *category, _Py_Identifier *id, const char *modname)
 {
+    PyObject *modname_obj = NULL;
     PyObject *action_str = _PyUnicode_FromId(id);
     if (action_str == NULL) {
         return NULL;
     }
 
+    /* Default to "no module name" for initial filter set */
+    if (modname != NULL) {
+        modname_obj = PyUnicode_InternFromString(modname);
+        if (modname_obj == NULL) {
+            return NULL;
+        }
+    } else {
+        modname_obj = Py_None;
+    }
+
     /* This assumes the line number is zero for now. */
     return PyTuple_Pack(5, action_str, Py_None,
-                        category, Py_None, _PyLong_Zero);
+                        category, modname_obj, _PyLong_Zero);
 }
 #endif
 
@@ -1180,20 +1204,22 @@ init_filters(void)
     return PyList_New(0);
 #else
     /* Other builds ignore a number of warning categories by default */
-    PyObject *filters = PyList_New(4);
+    PyObject *filters = PyList_New(5);
     if (filters == NULL) {
         return NULL;
     }
 
     size_t pos = 0;  /* Post-incremented in each use. */
     PyList_SET_ITEM(filters, pos++,
-                    create_filter(PyExc_DeprecationWarning, &PyId_ignore));
+                    create_filter(PyExc_DeprecationWarning, &PyId_default, "__main__"));
+    PyList_SET_ITEM(filters, pos++,
+                    create_filter(PyExc_DeprecationWarning, &PyId_ignore, NULL));
     PyList_SET_ITEM(filters, pos++,
-                    create_filter(PyExc_PendingDeprecationWarning, &PyId_ignore));
+                    create_filter(PyExc_PendingDeprecationWarning, &PyId_ignore, NULL));
     PyList_SET_ITEM(filters, pos++,
-                    create_filter(PyExc_ImportWarning, &PyId_ignore));
+                    create_filter(PyExc_ImportWarning, &PyId_ignore, NULL));
     PyList_SET_ITEM(filters, pos++,
-                    create_filter(PyExc_ResourceWarning, &PyId_ignore));
+                    create_filter(PyExc_ResourceWarning, &PyId_ignore, NULL));
 
     for (size_t x = 0; x < pos; x++) {
         if (PyList_GET_ITEM(filters, x) == NULL) {