identifiers.py: do not inherit from on-the-fly namedtuple

swh/model/identifiers.py

@@ -638,14 +638,17 @@ _object_type_map = {
 }
 
 
-class PersistentId(NamedTuple(
-        'PersistentId', [
-            ('namespace', str),
-            ('scheme_version', int),
-            ('object_type', str),
-            ('object_id', str),
-            ('metadata', Dict[str, Any]),
-        ])):
+_PersistentId = NamedTuple(
+    'PersistentId', [
+        ('namespace', str),
+        ('scheme_version', int),
+        ('object_type', str),
+        ('object_id', str),
+        ('metadata', Dict[str, Any]),
+    ])
+
+
+class PersistentId(_PersistentId):
     """
     Named tuple holding the relevant info associated to a Software Heritage
     persistent identifier.

swh/model/merkle.py

@@ -110,7 +110,7 @@ class MerkleNode(dict, metaclass=abc.ABCMeta):
     """
     __slots__ = ['parents', 'data', '__hash', 'collected']
 
-    type = None  # type: Optional[str]
+    type = None  # type: Optional[str]  # TODO: make this an enum
     """Type of the current node (used as a classifier for :func:`collect`)"""
 
     def __init__(self, data=None):