Add a method of Signed metadata class returning information about metadata expiration

tests/test_api.py

@@ -219,7 +219,25 @@ def test_metadata_base(self):
         md.signed.bump_expiration(timedelta(days=365))
         self.assertEqual(md.signed.expires, datetime(2031, 1, 2, 0, 0))
 
-
+        # Test is_expired with reference_time provided
+        is_expired = md.signed.is_expired(md.signed.expires)
+        self.assertTrue(is_expired)
+        is_expired = md.signed.is_expired(md.signed.expires + timedelta(days=1))
+        self.assertTrue(is_expired)
+        is_expired = md.signed.is_expired(md.signed.expires - timedelta(days=1))
+        self.assertFalse(is_expired)
+
+        # Test is_expired without reference_time, 
+        # manipulating md.signed.expires
+        expires = md.signed.expires
+        md.signed.expires = datetime.utcnow()
+        is_expired = md.signed.is_expired()
+        self.assertTrue(is_expired)
+        md.signed.expires = datetime.utcnow() + timedelta(days=1)
+        is_expired = md.signed.is_expired()
+        self.assertFalse(is_expired)
+        md.signed.expires = expires
+
     def test_metadata_snapshot(self):
         snapshot_path = os.path.join(
                 self.repo_dir, 'metadata', 'snapshot.json')

tuf/api/metadata.py

@@ -351,6 +351,22 @@ def _common_fields_to_dict(self) -> Dict[str, Any]:
             "expires": self.expires.isoformat() + "Z",
         }
 
+    def is_expired(self, reference_time: datetime = None) -> bool:
+        """Checks metadata expiration against a reference time.
+
+        Args:
+            reference_time: Optional; The time to check expiration date against.
+                A naive datetime in UTC expected.
+                If not provided, checks against the current UTC date and time.
+
+        Returns:
+            True if expiration time is less than the reference time.
+        """
+        if reference_time is None:
+            reference_time = datetime.utcnow()
+
+        return reference_time >= self.expires
+
     # Modification.
     def bump_expiration(self, delta: timedelta = timedelta(days=1)) -> None:
         """Increments the expires attribute by the passed timedelta. """