bpo-31040: Reject undotted extensions in mimetypes.add_type

OddBloke · OddBloke · commit 1485be9efe64 · 2019-09-10T09:33:01.000-04:00
Extensions that don't start with a dot will never be found by
methods/functions that act on the registry, so we should stop users from
mistakenly adding them.

The one exception is the empty string extension.  This could be in use
to detect absent extensions, so instead of raising a ValueError we emit
a warning.
diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
@@ -27,6 +27,7 @@
 import sys
 import posixpath
 import urllib.parse
+import warnings
 try:
     import winreg as _winreg
 except ImportError:
@@ -88,7 +89,15 @@ 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 not ext:
+            warnings.warn('Empty extension specified')
+        elif not ext.startswith('.'):
+            raise ValueError("Extensions should start with a '.'")
         self.types_map[strict][ext] = type
         exts = self.types_map_inv[strict].setdefault(type, [])
         if ext not in exts:
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
@@ -4,6 +4,7 @@
 import pathlib
 import sys
 import unittest
+import warnings
 
 from test import support
 from platform import win32_edition
@@ -168,6 +169,17 @@ def test_added_types_are_used(self):
         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):
diff --git a/Misc/NEWS.d/next/Library/2019-09-10-09-28-52.bpo-31040.VyAJS9.rst b/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.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+Reject undotted extensions in mimetypes.add_type.`