gh-125893: Add type check for category argument in warnings.simplefilter and warnings.filterwarning

Lib/_py_warnings.py

@@ -250,6 +250,19 @@ def _formatwarnmsg(msg):
                       msg.filename, msg.lineno, msg.line)
     return _wm._formatwarnmsg_impl(msg)
 
+def _valid_warning_category(category):
+    """
+    Return True if category is a Warning subclass or tuple of such.
+    Always perform class checks; only perform tuple iteration in debug mode.
+    """
+    if isinstance(category, type) and issubclass(category, Warning):
+        return True
+    if isinstance(category, tuple):
+        if __debug__:
+            return all(isinstance(c, type) and issubclass(c, Warning)
+                       for c in category)
+        return True
+    return False
 
 def filterwarnings(action, message="", category=Warning, module="", lineno=0,
                    append=False):
@@ -267,8 +280,9 @@ def filterwarnings(action, message="", category=Warning, module="", lineno=0,
         raise ValueError(f"invalid action: {action!r}")
     if not isinstance(message, str):
         raise TypeError("message must be a string")
-    if not isinstance(category, type) or not issubclass(category, Warning):
-        raise TypeError("category must be a Warning subclass")
+    if not _valid_warning_category(category):
+        raise TypeError("category must be a Warning subclass, "
+                        "not '{:s}'".format(type(category).__name__))
     if not isinstance(module, str):
         raise TypeError("module must be a string")
     if not isinstance(lineno, int):
@@ -303,6 +317,9 @@ def simplefilter(action, category=Warning, lineno=0, append=False):
     """
     if action not in {"error", "ignore", "always", "all", "default", "module", "once"}:
         raise ValueError(f"invalid action: {action!r}")
+    if not _valid_warning_category(category):
+        raise TypeError("category must be a Warning subclass, "
+                        "not '{:s}'".format(type(category).__name__))
     if not isinstance(lineno, int):
         raise TypeError("lineno must be an int")
     if lineno < 0:

Lib/test/test_warnings/__init__.py

@@ -398,6 +398,33 @@ def test_argument_validation(self):
         with self.assertRaises(ValueError):
             self.module.simplefilter('ignore', lineno=-1)
 
+    def test_invalid_category_types(self):
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category="notawarning")
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category=123)
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category=17.02)
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category=True)
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings(
+                "ignore", category=(UserWarning, 17)
+            )
+
+        with self.assertRaises(TypeError):
+            self.module.simplefilter("ignore", category="notawarning")
+        with self.assertRaises(TypeError):
+            self.module.simplefilter("ignore", category=123)
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category=17.02)
+        with self.assertRaises(TypeError):
+            self.module.filterwarnings("ignore", category=True)
+        with self.assertRaises(TypeError):
+            self.module.simplefilter(
+                "ignore", category=(UserWarning, 'abc')
+            )
+
     def test_catchwarnings_with_simplefilter_ignore(self):
         with self.module.catch_warnings(module=self.module):
             self.module.resetwarnings()

Misc/ACKS

@@ -75,6 +75,7 @@ Emmanuel Arias
 Alicia Arlen
 Jeffrey Armstrong
 Justin Turner Arthur
+Hasrat Ali Arzoo
 Jason Asbahr
 David Ascher
 Ammar Askar

Misc/NEWS.d/next/Windows/2025-07-10-19-28-11.gh-issue-125893.lD8Nyd.rst

@@ -0,0 +1,2 @@
+Added validation for the ``category`` argument in
+``warnings.filterwarnings()`` and ``warnings.simplefilter()``