API: Make _type read-only

tuf/api/metadata.py

@@ -324,19 +324,23 @@ class Signed:
 
     """
 
+    # Signed implementations are expected to override this
+    _signed_type = None
+
+    @property
+    def _type(self):
+        return self._signed_type
+
     # NOTE: Signed is a stupid name, because this might not be signed yet, but
     # we keep it to match spec terminology (I often refer to this as "payload",
     # or "inner metadata")
     def __init__(
         self,
-        _type: str,
         version: int,
         spec_version: str,
         expires: datetime,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-
-        self._type = _type
         self.spec_version = spec_version
         self.expires = expires
 
@@ -346,8 +350,8 @@ def __init__(
         self.version = version
         self.unrecognized_fields = unrecognized_fields or {}
 
-    @staticmethod
-    def _common_fields_from_dict(signed_dict: Mapping[str, Any]) -> list:
+    @classmethod
+    def _common_fields_from_dict(cls, signed_dict: Mapping[str, Any]) -> list:
         """Returns common fields of 'Signed' instances from the passed dict
         representation, and returns an ordered list to be passed as leading
         positional arguments to a subclass constructor.
@@ -356,14 +360,17 @@ def _common_fields_from_dict(signed_dict: Mapping[str, Any]) -> list:
 
         """
         _type = signed_dict.pop("_type")
+        if _type != cls._signed_type:
+            raise ValueError(f"Expected type {cls._signed_type}, got {_type}")
+
         version = signed_dict.pop("version")
         spec_version = signed_dict.pop("spec_version")
         expires_str = signed_dict.pop("expires")
         # Convert 'expires' TUF metadata string to a datetime object, which is
         # what the constructor expects and what we store. The inverse operation
         # is implemented in '_common_fields_to_dict'.
         expires = formats.expiry_string_to_datetime(expires_str)
-        return [_type, version, spec_version, expires]
+        return [version, spec_version, expires]
 
     def _common_fields_to_dict(self) -> Dict[str, Any]:
         """Returns dict representation of common fields of 'Signed' instances.
@@ -517,13 +524,14 @@ class Root(Signed):
 
     """
 
+    _signed_type = "root"
+
     # TODO: determine an appropriate value for max-args and fix places where
     # we violate that. This __init__ function takes 7 arguments, whereas the
     # default max-args value for pylint is 5
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        _type: str,
         version: int,
         spec_version: str,
         expires: datetime,
@@ -532,9 +540,7 @@ def __init__(
         roles: Mapping[str, Role],
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(
-            _type, version, spec_version, expires, unrecognized_fields
-        )
+        super().__init__(version, spec_version, expires, unrecognized_fields)
         self.consistent_snapshot = consistent_snapshot
         self.keys = keys
         self.roles = roles
@@ -612,18 +618,17 @@ class Timestamp(Signed):
 
     """
 
+    _signed_type = "timestamp"
+
     def __init__(
         self,
-        _type: str,
         version: int,
         spec_version: str,
         expires: datetime,
         meta: Mapping[str, Any],
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(
-            _type, version, spec_version, expires, unrecognized_fields
-        )
+        super().__init__(version, spec_version, expires, unrecognized_fields)
         # TODO: Add class for meta
         self.meta = meta
 
@@ -680,18 +685,17 @@ class Snapshot(Signed):
 
     """
 
+    _signed_type = "snapshot"
+
     def __init__(
         self,
-        _type: str,
         version: int,
         spec_version: str,
         expires: datetime,
         meta: Mapping[str, Any],
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(
-            _type, version, spec_version, expires, unrecognized_fields
-        )
+        super().__init__(version, spec_version, expires, unrecognized_fields)
         # TODO: Add class for meta
         self.meta = meta
 
@@ -782,23 +786,22 @@ class Targets(Signed):
 
     """
 
+    _signed_type = "targets"
+
     # TODO: determine an appropriate value for max-args and fix places where
     # we violate that. This __init__ function takes 7 arguments, whereas the
     # default max-args value for pylint is 5
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        _type: str,
         version: int,
         spec_version: str,
         expires: datetime,
         targets: Mapping[str, Any],
         delegations: Mapping[str, Any],
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
-        super().__init__(
-            _type, version, spec_version, expires, unrecognized_fields
-        )
+        super().__init__(version, spec_version, expires, unrecognized_fields)
         # TODO: Add class for meta
         self.targets = targets
         self.delegations = delegations