Metadata API: Don't do equality comparisons on containers

Use either "if X is not None:" or a try-except instead of a "if X:".

I believe Targets.from_dict() was not really broken with previous code
but it looks suspicious and did fail the added test with a strange
exception: I expect the from_dict() methods to mainly fail with
KeyErrors and ValueErrors if file format structure is incorrect.

Signed-off-by: Jussi Kukkonen <[email protected]>

tests/test_api.py

@@ -497,6 +497,14 @@ def test_delegation_class(self):
         delegations = Delegations.from_dict(copy.deepcopy(delegations_dict))
         self.assertEqual(delegations_dict, delegations.to_dict())
 
+        # Test some basic missing data
+        invalid_delegations_dicts = [
+            {},
+            {"keys":{}, "roles":[]},
+        ]
+        for d in invalid_delegations_dicts:
+            with self.assertRaises(KeyError):
+                Targets.from_dict(d)
 
     def test_metadata_targets(self):
         targets_path = os.path.join(

tuf/api/metadata.py

@@ -909,17 +909,20 @@ def from_dict(cls, targets_dict: Dict[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", None)
-        if delegations:
-            delegations = Delegations.from_dict(delegations)
+        try:
+            delegations_dict = targets_dict.pop("delegations")
+        except KeyError:
+            delegations = None
+        else:
+            delegations = Delegations.from_dict(delegations_dict)
         # 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["targets"] = self.targets
-        if self.delegations:
+        if self.delegations is not None:
             targets_dict["delegations"] = self.delegations.to_dict()
         return targets_dict