Return no match for empty supported media types

mimeparse/__init__.py

@@ -188,7 +188,7 @@ def best_match(supported: Iterable[str], header: str) -> str:
     conforms to the format of the HTTP Accept: header. The value of 'supported'
     is a list of mime-types. The list of supported mime-types should be sorted
     in order of increasing desirability, in case of a situation where there is
-    a tie.
+    a tie. Returns an empty string if no supported mime-type is acceptable.
 
     >>> best_match(['application/xbel+xml', 'text/xml'],
                    'text/*;q=0.5,*/*; q=0.1')
@@ -205,6 +205,8 @@ def best_match(supported: Iterable[str], header: str) -> str:
             mime_type
         ))
         pos += 1
+    if not weighted_matches:
+        return ''
     weighted_matches.sort()
 
     return weighted_matches[-1][0][0] and weighted_matches[-1][2] or ''

mimeparse_test.py

@@ -68,6 +68,22 @@ def test_best_match(self):
         for args, expected, description in self.test_data['best_match']:
             self._test_best_match(args, expected, description)
 
+    def test_best_match_empty_iterables(self):
+        for supported in ((), iter([]), (item for item in [])):
+            with self.subTest(supported=supported):
+                self.assertEqual(mimeparse.best_match(supported, '*/*'), '')
+
+    def test_best_match_generator(self):
+        supported = (item for item in ['text/plain', 'application/json'])
+        self.assertEqual(
+            mimeparse.best_match(supported, 'application/json'),
+            'application/json',
+        )
+
+    def test_best_match_empty_supported_invalid_header(self):
+        with self.assertRaises(mimeparse.MimeTypeParseException):
+            mimeparse.best_match([], 'invalid')
+
     def test_parse_mime_type(self):
         for args, expected in self.test_data['parse_mime_type']:
             self._test_parse_mime_type(args, expected)

testdata.json

@@ -66,6 +66,14 @@
     ],
 
     "best_match": [
+        [
+            [
+                [],
+                "application/json"
+            ],
+            "",
+            "no match when no media types are supported"
+        ],
         [
             [
                 ["application/json", "text/html"],