Update Link.is_hash_allowed() to accept hashes=None (#6774)

Author: pradyunsg

news/6772.bugfix

@@ -0,0 +1,2 @@
+Fix a ``NoneType`` ``AttributeError`` when evaluating hashes and no hashes
+are provided.

src/pip/_internal/models/link.py

@@ -200,11 +200,11 @@ def has_hash(self):
         return self.hash_name is not None
 
     def is_hash_allowed(self, hashes):
-        # type: (Hashes) -> bool
+        # type: (Optional[Hashes]) -> bool
         """
         Return True if the link has a hash and it is allowed.
         """
-        if not self.has_hash:
+        if hashes is None or not self.has_hash:
             return False
         # Assert non-None so mypy knows self.hash_name and self.hash are str.
         assert self.hash_name is not None

tests/unit/test_link.py

@@ -117,3 +117,13 @@ def test_is_hash_allowed__no_hash(self):
         }
         hashes = Hashes(hashes_data)
         assert not link.is_hash_allowed(hashes)
+
+    @pytest.mark.parametrize('hashes, expected', [
+        (None, False),
+        # Also test a success case to show the test is correct.
+        (Hashes({'sha512': [128 * 'a']}), True),
+    ])
+    def test_is_hash_allowed__none_hashes(self, hashes, expected):
+        url = 'https://example.com/wheel.whl#sha512={}'.format(128 * 'a')
+        link = Link(url)
+        assert link.is_hash_allowed(hashes) == expected