Clarifications and simplifications

Clarify explicitly that exactly one of "paths" and "path_hash_prefixes"
must be set inside DelegatedRole.
Also simplify the check for "paths" and "path_hash_prefixes".
Finally, add a test case inside the "test_metadata_serialization.py"
test file about wrong keyids type for "Role" serialization.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Author: Martin Vrachev

tests/test_metadata_serialization.py

@@ -176,6 +176,7 @@ def test_invalid_key_serialization(self, test_case_data: str) -> None:
         "no threshold": '{"keyids": ["keyid"]}',
         "no keyids": '{"threshold": 3}',
         "wrong threshold type": '{"keyids": ["keyid"], "threshold": "a"}',
+        "wrong keyids type": '{"keyids": 1, "threshold": 3}',
         "threshold below 1": '{"keyids": ["keyid"], "threshold": 0}',
         "duplicate keyids": '{"keyids": ["keyid", "keyid"], "threshold": 3}',
     }

tuf/api/metadata.py

@@ -1297,7 +1297,7 @@ class DelegatedRole(Role):
         paths: Path patterns. See note above.
         path_hash_prefixes: Hash prefixes. See note above.
         unrecognized_fields: Dictionary of all attributes that are not managed
-            by TUF Metadata API
+            by TUF Metadata API.
 
     Raises:
         ValueError: Invalid arguments.
@@ -1316,11 +1316,11 @@ def __init__(
         super().__init__(keyids, threshold, unrecognized_fields)
         self.name = name
         self.terminating = terminating
-        if paths is not None and path_hash_prefixes is not None:
-            raise ValueError("Either paths or path_hash_prefixes can be set")
-
-        if paths is None and path_hash_prefixes is None:
-            raise ValueError("One of paths or path_hash_prefixes must be set")
+        exclusive_vars = [paths, path_hash_prefixes]
+        if sum(1 for var in exclusive_vars if var is not None) != 1:
+            raise ValueError(
+                "Only one of (paths, path_hash_prefixes) must be set"
+            )
 
         if paths is not None and any(not isinstance(p, str) for p in paths):
             raise ValueError("Paths must be strings")
@@ -1349,7 +1349,7 @@ def from_dict(cls, role_dict: Dict[str, Any]) -> "DelegatedRole":
         """Creates ``DelegatedRole`` object from its json/dict representation.
 
         Raises:
-            ValueError, KeyError: Invalid arguments.
+            ValueError, KeyError, TypeError: Invalid arguments.
         """
         name = role_dict.pop("name")
         keyids = role_dict.pop("keyids")