Make Metadata class Generic

'Generic' is used to help type checkers and IDEs correctly
understand the 'signed' attribute type which is discovered
runtime.
This is entirely a typing feature and has no runtime effect
on the existing code.

Co-authored-by: Jussi Kukkonen <jkukkonen@vmware.com>
Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/api/metadata.py

@@ -26,12 +26,15 @@
     Any,
     ClassVar,
     Dict,
+    Generic,
     List,
     Mapping,
     Optional,
     Tuple,
     Type,
+    TypeVar,
     Union,
+    cast,
 )
 
 from securesystemslib import exceptions as sslib_exceptions
@@ -56,8 +59,10 @@
 # files to have the same major version (the first number) as ours.
 SPECIFICATION_VERSION = ["1", "0", "19"]
 
+T = TypeVar("T", "Root", "Timestamp", "Snapshot", "Targets")
 
-class Metadata:
+
+class Metadata(Generic[T]):
     """A container for signed TUF metadata.
 
     Provides methods to convert to and from dictionary, read and write to and
@@ -70,10 +75,8 @@ class Metadata:
             signing the canonical serialized representation of 'signed'.
     """
 
-    def __init__(
-        self, signed: "Signed", signatures: "OrderedDict[str, Signature]"
-    ):
-        self.signed = signed
+    def __init__(self, signed: T, signatures: "OrderedDict[str, Signature]"):
+        self.signed: T = signed
         self.signatures = signatures
 
     @classmethod
@@ -119,7 +122,8 @@ def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata":
             signatures[sig.keyid] = sig
 
         return cls(
-            signed=inner_cls.from_dict(metadata.pop("signed")),
+            # 'cast' is used only to help typing, it has no runtime effect
+            signed=cast(T, inner_cls.from_dict(metadata.pop("signed"))),
             signatures=signatures,
         )
 

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -71,7 +71,7 @@
 from typing import Dict, Iterator, Mapping, Optional
 
 from tuf import exceptions
-from tuf.api.metadata import Key, Metadata, Root, Targets
+from tuf.api.metadata import Key, Metadata, Root, Snapshot, Targets, Timestamp
 from tuf.api.serialization import DeserializationError
 
 logger = logging.getLogger(__name__)
@@ -156,22 +156,22 @@ def __iter__(self) -> Iterator[Metadata]:
 
     # Helper properties for top level metadata
     @property
-    def root(self) -> Metadata:
+    def root(self) -> Metadata[Root]:
         """Current root Metadata"""
         return self._trusted_set["root"]
 
     @property
-    def timestamp(self) -> Optional[Metadata]:
+    def timestamp(self) -> Optional[Metadata[Timestamp]]:
         """Current timestamp Metadata or None"""
         return self._trusted_set.get("timestamp")
 
     @property
-    def snapshot(self) -> Optional[Metadata]:
+    def snapshot(self) -> Optional[Metadata[Snapshot]]:
         """Current snapshot Metadata or None"""
         return self._trusted_set.get("snapshot")
 
     @property
-    def targets(self) -> Optional[Metadata]:
+    def targets(self) -> Optional[Metadata[Targets]]:
         """Current targets Metadata or None"""
         return self._trusted_set.get("targets")
 
@@ -196,7 +196,7 @@ def update_root(self, data: bytes) -> None:
         logger.debug("Updating root")
 
         try:
-            new_root = Metadata.from_bytes(data)
+            new_root = Metadata[Root].from_bytes(data)
         except DeserializationError as e:
             raise exceptions.RepositoryError("Failed to load root") from e
 
@@ -261,7 +261,7 @@ def update_timestamp(self, data: bytes) -> None:
             raise RuntimeError("Cannot update timestamp after snapshot")
 
         try:
-            new_timestamp = Metadata.from_bytes(data)
+            new_timestamp = Metadata[Timestamp].from_bytes(data)
         except DeserializationError as e:
             raise exceptions.RepositoryError("Failed to load timestamp") from e
 
@@ -330,7 +330,7 @@ def update_snapshot(self, data: bytes) -> None:
             ) from e
 
         try:
-            new_snapshot = Metadata.from_bytes(data)
+            new_snapshot = Metadata[Snapshot].from_bytes(data)
         except DeserializationError as e:
             raise exceptions.RepositoryError("Failed to load snapshot") from e
 
@@ -429,7 +429,7 @@ def update_delegated_targets(
             ) from e
 
         try:
-            new_delegate = Metadata.from_bytes(data)
+            new_delegate = Metadata[Targets].from_bytes(data)
         except DeserializationError as e:
             raise exceptions.RepositoryError("Failed to load snapshot") from e