gh-75223: Reject undotted extensions in mimetypes.add_type()

Doc/library/mimetypes.rst

@@ -307,10 +307,6 @@ than one MIME-type database; it provides an interface similar to the one of the
       When *strict* is ``True`` (the default), the mapping will be added to the
       official MIME types, otherwise to the non-standard ones.
 
-      .. deprecated-removed:: 3.14 3.16
-         Invalid, undotted extensions will raise a
-         :exc:`ValueError` in Python 3.16.
-
 
 .. _mimetypes-cli:
 

Doc/whatsnew/3.16.rst

@@ -120,6 +120,13 @@ functools
 * Calling the Python implementation of :func:`functools.reduce` with *function*
   or *sequence* as keyword arguments has been deprecated since Python 3.14.
 
+mimetypes
+---------
+
+* Passing an invalid *ext* (that is, an extension that is not empty and does not
+  start with a '.') to :meth:`mimetypes.MimeTypes.add_type` now raises a :exc:`ValueError`.
+  (Contributed by Daniel Watkins, Hugo van Kemenade, and Stan Ulbrych in :gh:`75223`.)
+
 symtable
 --------
 

Lib/mimetypes.py

@@ -93,14 +93,7 @@ def add_type(self, type, ext, strict=True):
         Valid extensions are empty or start with a '.'.
         """
         if ext and not ext.startswith('.'):
-            from warnings import _deprecated
-
-            _deprecated(
-                "Undotted extensions",
-                "Using undotted extensions is deprecated and "
-                "will raise a ValueError in Python {remove}",
-                remove=(3, 16),
-            )
+            raise ValueError("Extensions should start with a '.' or be empty")
 
         if not type:
             return

Lib/test/test_mimetypes.py

@@ -389,9 +389,11 @@ def test_added_types_are_used(self):
         mime_type, _ = mimetypes.guess_type('test.myext')
         self.assertEqual(mime_type, 'testing/type')
 
-    def test_add_type_with_undotted_extension_deprecated(self):
-        with self.assertWarns(DeprecationWarning):
-            mimetypes.add_type("testing/type", "undotted")
+    def test_add_type_with_undotted_extension_raises_exception(self):
+        with self.assertRaisesRegex(
+            ValueError, "Extensions should start with a '.' or be empty"
+        ):
+            mimetypes.add_type('testing/type', 'undotted')
 
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")

Misc/NEWS.d/next/Library/2019-09-10-09-28-52.gh-issue-75223.VyAJS9.rst

@@ -0,0 +1 @@
+Reject undotted extensions in :meth:`mimetypes.MimeTypes.add_type`.