swh.model.hashutil: Simplify length hash algorithms instantiation

The same caveat applies, will only be supported from python3.6 onward.

Related T692

swh/model/hashutil.py

@@ -84,14 +84,7 @@ def _new_hash(algo, length=None):
             base_algo = _algo[0]
             variable_length = int(_algo[1])
 
-            if base_algo == 'blake2b':
-                h = hashlib.blake2b(digest_size=variable_length)
-            elif base_algo == 'blake2s':
-                h = hashlib.blake2s(digest_size=variable_length)
-            else:
-                raise ValueError('Unexpected hashing algorithm %s, '
-                                 'expected one of %s' %
-                                 (algo, ', '.join(sorted(ALGORITHMS))))
+            h = hashlib.new('%s%s' % (base_algo, variable_length))
         else:
             raise ValueError('Unsupported hashing algorithm %s' % algo)
     else:

swh/model/tests/test_hashutil.py

@@ -142,32 +142,32 @@ class Hashutil(unittest.TestCase):
             hashutil._new_hash('blake3:256')
         except ValueError as e:
             self.assertEquals(str(e),
-                              'Unexpected hashing algorithm blake3:256, '
-                              'expected one of sha1, sha1_git, sha256')
+                              'unsupported hash type blake3256')
 
     @patch('swh.model.hashutil.sys')
     @patch('swh.model.hashutil.hashlib')
     @istest
     def new_hash_blake2b(self, mock_hashlib, mock_sys):
         mock_sys.version_info = MagicMock(major=3, minor=6)
-        mock_hashlib.blake2b.return_value = 'some-hashlib-object'
+        mock_hashlib.new.return_value = 'some-hashlib-object'
 
-        h = hashutil._new_hash('blake2b:256')
+        h = hashutil._new_hash('blake2b:512')
 
         self.assertEquals(h, 'some-hashlib-object')
-        mock_hashlib.blake2b.assert_called_with(digest_size=256)
+        mock_hashlib.new.assert_called_with('blake2b512')
 
     @patch('swh.model.hashutil.sys')
     @patch('swh.model.hashutil.hashlib')
     @istest
     def new_hash_blake2s(self, mock_hashlib, mock_sys):
         mock_sys.version_info = MagicMock(major=3, minor=6)
-        mock_hashlib.blake2s.return_value = 'some-hashlib-object'
+        mock_hashlib.new.return_value = 'some-hashlib-object'
 
-        h = hashutil._new_hash('blake2s:128')
+        h = hashutil._new_hash('blake2s:256')
 
         self.assertEquals(h, 'some-hashlib-object')
         mock_hashlib.blake2s.assert_called_with(digest_size=128)
+        mock_hashlib.new.assert_called_with('blake2s256')
 
 
 class HashlibGit(unittest.TestCase):