LanguageAccept falls back to language only

src/werkzeug/datastructures.py

@@ -1849,6 +1849,19 @@ def _normalize(language):
 
         return item == "*" or _normalize(value) == _normalize(item)
 
+    def best_match(self, matches, default=None):
+        """Calls the super version of `best_match` and if it returns none,
+        attempts to fall back to language only matches.
+
+        :param matches: a list of matches to check for
+        :param default: the value that is returned if none match
+        """
+        result = super(LanguageAccept, self).best_match(matches)
+        if result is not None:
+            return result
+        fallback = Accept([(item[0][0:2], item[1]) for item in self])
+        return fallback.best_match(matches, default=default)
+
 
 class CharsetAccept(Accept):
     """Like :class:`Accept` but with normalization for charsets."""

tests/test_datastructures.py

@@ -1074,6 +1074,35 @@ def test_accept_wildcard_specificity(self):
         assert accept.best_match(["text/plain", "image/png"]) == "image/png"
 
 
+class TestLanguageAccept(object):
+    storage_class = datastructures.LanguageAccept
+
+    def test_best_match_fallback(self):
+        accept = self.storage_class([("en-us", 1)])
+        assert accept.best_match(["en"]) == "en"
+
+        accept = self.storage_class([("de_AT", 1), ("de", 0.9)])
+        assert accept.best_match(["en"]) is None
+
+        accept = self.storage_class([("de_AT", 1), ("de", 0.9), ("en-US", 0.8)])
+        assert accept.best_match(["de", "en"]) == "de"
+
+        accept = self.storage_class([("de_AT", 0.9), ("en-US", 1)])
+        assert accept.best_match(["en"]) == "en"
+
+        accept = self.storage_class([("en-us", 1)])
+        assert accept.best_match(["en-us"]) == "en-us"
+
+        accept = self.storage_class([("en-us", 1)])
+        assert accept.best_match(["en-us", "en"]) == "en-us"
+
+        accept = self.storage_class([("en-GB", 1)])
+        assert accept.best_match(["en-US", "en"], default="en-US") == "en"
+
+        accept = self.storage_class([("de-AT", 1)])
+        assert accept.best_match(["en-US", "en"], default="en-US") == "en-US"
+
+
 class TestFileStorage(object):
     storage_class = datastructures.FileStorage