BREAKING CHANGE: Make delegations optional

According to the spec, delegations in targets are marked as optional:
https://theupdateframework.github.io/specification/latest/#file-formats-targets
and a pr, clarifying that even more, is approved:
theupdateframework/specification#157.

This is a possible breaking change.

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

Author: MVrachev

tests/test_api.py

@@ -375,6 +375,14 @@ def test_metadata_targets(self):
         # Verify that data is updated
         self.assertEqual(targets.signed.targets[filename], fileinfo)
 
+        # Test from_dict/to_dict Targets without delegations
+        targets_dict = targets.to_dict()
+        del targets_dict["signed"]["delegations"]
+        tmp_dict = targets_dict["signed"].copy()
+        targets_obj = Targets.from_dict(tmp_dict)
+        tar_d = targets_obj.to_dict()
+        self.assertEqual(targets_dict["signed"], targets_obj.to_dict())
+
     def setup_dict_with_unrecognized_field(self, file_path, field, value):
         json_dict = {}
         with open(file_path) as f:

tuf/api/metadata.py

@@ -717,7 +717,7 @@ def __init__(
         spec_version: str,
         expires: datetime,
         targets: Mapping[str, Any],
-        delegations: Mapping[str, Any],
+        delegations: Optional[Mapping[str, Any]] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
         super().__init__(
@@ -732,19 +732,16 @@ def from_dict(cls, targets_dict: Mapping[str, Any]) -> "Targets":
         """Creates Targets object from its dict representation."""
         common_args = cls._common_fields_from_dict(targets_dict)
         targets = targets_dict.pop("targets")
-        delegations = targets_dict.pop("delegations")
+        delegations = targets_dict.pop("delegations", None)
         # All fields left in the targets_dict are unrecognized.
         return cls(*common_args, targets, delegations, targets_dict)
 
     def to_dict(self) -> Dict[str, Any]:
         """Returns the dict representation of self."""
         targets_dict = self._common_fields_to_dict()
-        targets_dict.update(
-            {
-                "targets": self.targets,
-                "delegations": self.delegations,
-            }
-        )
+        targets_dict["targets"] = self.targets
+        if self.delegations:
+            targets_dict["delegations"] = self.delegations
         return targets_dict
 
     # Modification.