From: sobolevn <mail@sobolevn.me>
Date: Tue, 12 May 2026 13:40:21 +0000 (+0300)
Subject: gh-149720: Remove support for undotted `ext` in `mimetypes.MimeType.add_type` (#149721)
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=0534774a472424f6b9be2dc4ce9784384bc02401;p=thirdparty%2FPython%2Fcpython.git

gh-149720: Remove support for undotted `ext` in `mimetypes.MimeType.add_type` (#149721)
---

diff --git a/Doc/library/mimetypes.rst b/Doc/library/mimetypes.rst
index 0facacd50fd3..eed9acb92e0c 100644
--- a/Doc/library/mimetypes.rst
+++ b/Doc/library/mimetypes.rst
@@ -129,10 +129,18 @@ behavior of the module.
    Add a mapping from the MIME type *type* to the extension *ext*. When the
    extension is already known, the new type will replace the old one. When the type
    is already known the extension will be added to the list of known extensions.
+   Valid extensions are empty or start with a ``'.'``.
 
    When *strict* is ``True`` (the default), the mapping will be added to the
    official MIME types, otherwise to the non-standard ones.
 
+   .. deprecated:: 3.14
+      *ext* values that do not start with ``'.'`` are deprecated.
+
+   .. versionchanged:: next
+      *ext* now must start with ``'.'``. Otherwise :exc:`ValueError` is raised.
+
+
 
 .. data:: inited
 
diff --git a/Doc/whatsnew/3.16.rst b/Doc/whatsnew/3.16.rst
index 6d91d53f478d..f79f976ee55c 100644
--- a/Doc/whatsnew/3.16.rst
+++ b/Doc/whatsnew/3.16.rst
@@ -127,6 +127,13 @@ logging
   and scheduled for removal in Python 3.16. Define handlers with the *stream*
   argument instead.
 
+mimetypes
+---------
+
+* Valid extensions start with a '.' or are empty for
+  :meth:`mimetypes.MimeTypes.add_type`.
+  Undotted extensions now raise a :exc:`ValueError`.
+
 symtable
 --------
 
diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index 6d9278bccf92..15e8c0a437bf 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -93,14 +93,7 @@ class MimeTypes:
         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(f"Extension {ext!r} must start with '.'")
 
         if not type:
             return
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index 2d618081521e..607aff8418ed 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -389,9 +389,12 @@ class MimeTypesTestCase(unittest.TestCase):
         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):
+    def test_add_type_with_undotted_extension_not_supported(self):
+        msg = "Extension 'undotted' must start with '.'"
+        with self.assertRaisesRegex(ValueError, msg):
             mimetypes.add_type("testing/type", "undotted")
+        with self.assertRaisesRegex(ValueError, msg):
+            mimetypes.add_type("", "undotted")
 
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
diff --git a/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst b/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst
new file mode 100644
index 000000000000..f16d8770d740
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst
@@ -0,0 +1 @@
+Remove support for undotted *ext* in :meth:`mimetypes.MimeTypes.add_type`.