bpo-4963: Fix for initialization and non-deterministic behavior issues in mimetypes (GH-3062)

Doc/library/mimetypes.rst

@@ -93,6 +93,10 @@ behavior of the module.
    Specifying an empty list for *files* will prevent the system defaults from
    being applied: only the well-known values will be present from a built-in list.
 
+   If *files* is ``None`` the internal data structure is completely rebuilt to its
+   initial default value. This is a stable operation and will produce the same results
+   when called multiple times.
+
    .. versionchanged:: 3.2
       Previously, Windows registry settings were ignored.
 

Lib/test/test_mimetypes.py

@@ -79,6 +79,57 @@ class MimeTypesTestCase(unittest.TestCase):
                                           strict=True)
         self.assertEqual(exts, ['.g3', '.g\xb3'])
 
+    def test_init_reinitializes(self):
+        # Issue 4936: make sure an init starts clean
+        # First, put some poison into the types table
+        mimetypes.add_type('foo/bar', '.foobar')
+        self.assertEqual(mimetypes.guess_extension('foo/bar'), '.foobar')
+        # Reinitialize
+        mimetypes.init()
+        # Poison should be gone.
+        self.assertEqual(mimetypes.guess_extension('foo/bar'), None)
+
+    def test_preferred_extension(self):
+        def check_extensions():
+            self.assertEqual(mimetypes.guess_extension('application/octet-stream'), '.bin')
+            self.assertEqual(mimetypes.guess_extension('application/postscript'), '.ps')
+            self.assertEqual(mimetypes.guess_extension('application/vnd.apple.mpegurl'), '.m3u')
+            self.assertEqual(mimetypes.guess_extension('application/vnd.ms-excel'), '.xls')
+            self.assertEqual(mimetypes.guess_extension('application/vnd.ms-powerpoint'), '.ppt')
+            self.assertEqual(mimetypes.guess_extension('application/x-texinfo'), '.texi')
+            self.assertEqual(mimetypes.guess_extension('application/x-troff'), '.roff')
+            self.assertEqual(mimetypes.guess_extension('application/xml'), '.xsl')
+            self.assertEqual(mimetypes.guess_extension('audio/mpeg'), '.mp3')
+            self.assertEqual(mimetypes.guess_extension('image/jpeg'), '.jpg')
+            self.assertEqual(mimetypes.guess_extension('image/tiff'), '.tiff')
+            self.assertEqual(mimetypes.guess_extension('message/rfc822'), '.eml')
+            self.assertEqual(mimetypes.guess_extension('text/html'), '.html')
+            self.assertEqual(mimetypes.guess_extension('text/plain'), '.txt')
+            self.assertEqual(mimetypes.guess_extension('video/mpeg'), '.mpeg')
+            self.assertEqual(mimetypes.guess_extension('video/quicktime'), '.mov')
+
+        check_extensions()
+        mimetypes.init()
+        check_extensions()
+
+    def test_init_stability(self):
+        mimetypes.init()
+
+        suffix_map = mimetypes.suffix_map
+        encodings_map = mimetypes.encodings_map
+        types_map = mimetypes.types_map
+        common_types = mimetypes.common_types
+
+        mimetypes.init()
+        self.assertIsNot(suffix_map, mimetypes.suffix_map)
+        self.assertIsNot(encodings_map, mimetypes.encodings_map)
+        self.assertIsNot(types_map, mimetypes.types_map)
+        self.assertIsNot(common_types, mimetypes.common_types)
+        self.assertEqual(suffix_map, mimetypes.suffix_map)
+        self.assertEqual(encodings_map, mimetypes.encodings_map)
+        self.assertEqual(types_map, mimetypes.types_map)
+        self.assertEqual(common_types, mimetypes.common_types)
+
     def test_path_like_ob(self):
         filename = "LICENSE.txt"
         filepath = pathlib.Path(filename)

Misc/NEWS.d/next/Library/2017-08-15-11-24-41.bpo-4963.LRYres.rst

+Fixed non-deterministic behavior related to mimetypes extension mapping and
+module reinitialization.