pythongh-112451: Deprecate subclassing of pure-Python datetime.timezone.

This is consistent with C-extension datetime.timezone.

Lib/_pydatetime.py

@@ -2347,6 +2347,18 @@ def __new__(cls, offset, name=_Omitted):
                              "timedelta(hours=24).")
         return cls._create(offset, name)
 
+    def __init_subclass__(cls):
+        # When deprecation ends replace with:
+        #   raise TypeError("type 'datetime.timezone' is not an acceptable base type")
+        import warnings
+        warnings.warn(
+            "Subclassing 'datetime.timezone' is deprecated and scheduled for removal "
+            "in Python 3.15.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        super().__init_subclass__()
+
     @classmethod
     def _create(cls, offset, name=None):
         self = tzinfo.__new__(cls)

Lib/test/datetimetester.py

@@ -301,6 +301,25 @@ def test_inheritance(self):
         self.assertIsInstance(timezone.utc, tzinfo)
         self.assertIsInstance(self.EST, tzinfo)
 
+    def test_cannot_subclass_deprecation_warning(self):
+        if '_Fast' in self.__class__.__name__:
+            self.skipTest('Only run for Pure Python implementation')
+
+        msg = (
+            "Subclassing 'datetime.timezone' is deprecated and scheduled for removal "
+            "in Python 3.15."
+        )
+        with self.assertWarnsRegex(DeprecationWarning, msg):
+            class MyTimezone(timezone): pass
+
+    def test_cannot_subclass(self):
+        if '_Pure' in self.__class__.__name__:
+            self.skipTest('Only run for Fast C implementation')
+
+        msg = "type 'datetime.timezone' is not an acceptable base type"
+        with self.assertRaisesRegex(TypeError, msg):
+            class MyTimezone(timezone): pass
+
     def test_utcoffset(self):
         dummy = self.DT
         for h in [0, 1.5, 12]:

Misc/NEWS.d/next/Library/2024-01-17-16-34-00.gh-issue-112451.5y9enV.rst

@@ -0,0 +1,2 @@
+Deprecate subclassing pure-Python ``datetime.timezone``.
+This is consistent with C-extension implementation.