warnings.filters becomes an immutable tuple

Make it more explicit that warnings.filters must not be replaced
directly, but modified by:

* filterwarning()
* simplefilter()
* resetwarnings()

Lib/test/test_warnings/__init__.py

@@ -850,15 +850,14 @@ def test_issue31416(self):
         # bad warnings.filters or warnings.defaultaction.
         wmod = self.module
         with original_warnings.catch_warnings(module=wmod):
-            wmod.filters = [(None, None, Warning, None, 0)]
             with self.assertRaises(TypeError):
-                wmod._filters_mutated()
+                wmod._add_filter(None, None, Warning, None, 0)
                 wmod.warn_explicit('foo', Warning, 'bar', 1)
 
-            wmod.filters = []
+        with original_warnings.catch_warnings(module=wmod):
             with support.swap_attr(wmod, 'defaultaction', None), \
                  self.assertRaises(TypeError):
-                wmod._filters_mutated()
+                wmod.resetwarnings()
                 wmod.warn_explicit('foo', Warning, 'bar', 1)
 
     @support.cpython_only
@@ -1016,14 +1015,14 @@ def test_catch_warnings_defaults(self):
         with wmod.catch_warnings(module=wmod) as w:
             self.assertIsNone(w)
             self.assertIs(wmod.showwarning, orig_showwarning)
-            self.assertIsNot(wmod.filters, orig_filters)
+            self.assertIs(wmod.filters, orig_filters)
         self.assertIs(wmod.filters, orig_filters)
         if wmod is sys.modules['warnings']:
             # Ensure the default module is this one
             with wmod.catch_warnings() as w:
                 self.assertIsNone(w)
                 self.assertIs(wmod.showwarning, orig_showwarning)
-                self.assertIsNot(wmod.filters, orig_filters)
+                self.assertIs(wmod.filters, orig_filters)
             self.assertIs(wmod.filters, orig_filters)
 
     def test_record_override_showwarning_before(self):

Lib/warnings.py

@@ -165,22 +165,26 @@ def simplefilter(action, category=Warning, lineno=0, append=False):
     _add_filter(action, None, category, None, lineno, append=append)
 
 def _add_filter(*item, append):
+    global filters
     # Remove possible duplicate filters, so new one will be placed
     # in correct place. If append=True and duplicate exists, do nothing.
+    filters_list = list(filters)
     if not append:
         try:
-            filters.remove(item)
+            filters_list.remove(item)
         except ValueError:
             pass
-        filters.insert(0, item)
+        filters_list.insert(0, item)
     else:
-        if item not in filters:
-            filters.append(item)
+        if item not in filters_list:
+            filters_list.append(item)
+    filters = tuple(filters_list)
     _filters_mutated()
 
 def resetwarnings():
     """Clear the list of warning filters, so that no filters are active."""
-    filters[:] = []
+    global filters
+    filters = ()
     _filters_mutated()
 
 class _OptionError(Exception):
@@ -462,7 +466,7 @@ def __enter__(self):
             raise RuntimeError("Cannot enter %r twice" % self)
         self._entered = True
         self._filters = self._module.filters
-        self._module.filters = self._filters[:]
+        self._module.filters = tuple(self._filters)
         self._module._filters_mutated()
         self._showwarning = self._module.showwarning
         self._showwarnmsg_impl = self._module._showwarnmsg_impl
@@ -499,8 +503,9 @@ def __exit__(self, *exc_info):
     defaultaction = _defaultaction
     onceregistry = _onceregistry
     _warnings_defaults = True
+    filters = tuple(filters)
 except ImportError:
-    filters = []
+    filters = ()
     defaultaction = "default"
     onceregistry = {}
 

Python/_warnings.c

@@ -271,10 +271,10 @@ get_filters(void)
         Py_SETREF(_PyRuntime.warnings.filters, warnings_filters);
     }
 
-    PyObject *filters_list = _PyRuntime.warnings.filters;
-    if (filters_list == NULL || !PyList_Check(filters_list)) {
+    PyObject *filters_tuple = _PyRuntime.warnings.filters;
+    if (filters_tuple == NULL || !PyTuple_Check(filters_tuple)) {
         PyErr_SetString(PyExc_ValueError,
-                        MODULE_NAME ".filters must be a list");
+                        MODULE_NAME ".filters must be a tuple");
         return NULL;
     }
 
@@ -283,8 +283,9 @@ get_filters(void)
     new_filters.version = _PyRuntime.warnings.filters_version;
 
     /* _PyRuntime.warnings.filters could change while we are iterating over it. */
-    for (Py_ssize_t i = 0; i < PyList_GET_SIZE(filters_list); i++) {
-        PyObject *item = PyList_GET_ITEM(filters_list, i);
+    Py_ssize_t nfilter = PyTuple_GET_SIZE(filters_tuple);
+    for (Py_ssize_t i = 0; i < nfilter; i++) {
+        PyObject *item = PyTuple_GET_ITEM(filters_tuple, i);
         if (!PyTuple_Check(item) || PyTuple_GET_SIZE(item) != 5) {
             PyErr_Format(PyExc_ValueError,
                          MODULE_NAME ".filters item %zd isn't a 5-tuple", i);
@@ -1408,7 +1409,10 @@ init_filters(void)
         }
     }
 
-    return filters;
+    PyObject *filters_tuple = PyList_AsTuple(filters);
+    Py_DECREF(filters);
+
+    return filters_tuple;
 }
 
 static struct PyModuleDef warningsmodule = {