gh-75223: Reject undotted extensions in mimetypes.add_type

Lib/mimetypes.py

@@ -27,6 +27,7 @@
 import sys
 import posixpath
 import urllib.parse
+import warnings
 
 try:
     from _winapi import _mimetypes_read_windows_registry
@@ -94,7 +95,13 @@ def add_type(self, type, ext, strict=True):
         If strict is true, information will be added to
         list of standard types, else to the list of non-standard
         types.
+
+        Non-empty extensions that don't start with a '.' are not
+        valid, so a ValueError will be raised if they are
+        specified.
         """
+        if ext and not ext.startswith('.'):
+            raise ValueError("Extensions should start with a '.' or be empty")
         self.types_map[strict][ext] = type
         exts = self.types_map_inv[strict].setdefault(type, [])
         if ext not in exts:

Lib/test/test_mimetypes.py

@@ -3,6 +3,7 @@
 import os
 import sys
 import unittest.mock
+import warnings
 
 from test import support
 from test.support import os_helper
@@ -305,6 +306,24 @@ def test_keywords_args_api(self):
         self.assertEqual(self.db.guess_extension(
             type='image/jpg', strict=False), '.jpg')
 
+    def test_added_types_are_used(self):
+        mime_type, _ = mimetypes.guess_type('test.myext')
+        self.assertEqual(None, mime_type)
+        mimetypes.add_type('testing/type', '.myext')
+        mime_type, _ = mimetypes.guess_type('test.myext')
+        self.assertEqual('testing/type', mime_type)
+
+    def test_add_type_with_undotted_extension_raises_exception(self):
+        with self.assertRaises(ValueError):
+            mimetypes.add_type('testing/type', 'undotted')
+
+    def test_add_type_with_empty_extension_emits_warning(self):
+        with warnings.catch_warnings(record=True) as wlog:
+            mimetypes.add_type('testing/type', '')
+        self.assertEqual(1, len(wlog))
+        warning = wlog[0]
+        self.assertEqual('Empty extension specified', str(warning.message))
+
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):

Misc/NEWS.d/next/Library/2019-09-10-09-28-52.bpo-31040.VyAJS9.rst

@@ -0,0 +1 @@
+Reject undotted extensions in mimetypes.add_type.