Add from_json metadata convenience wrapper

Add convenience wrapper that takes a json string and passes it
to from_dict to create a Metadata object.

Signed-off-by: Lukas Puehringer <[email protected]>

Author: lukpueh

tests/test_api.py

@@ -89,13 +89,25 @@ def test_generic_read(self):
                 ('timestamp', Timestamp),
                 ('targets', Targets)]:
 
+            # Load JSON-formatted metdata of each supported type from file
+            # and from out-of-band read JSON string
             path = os.path.join(self.repo_dir, 'metadata', metadata + '.json')
             metadata_obj = Metadata.from_json_file(path)
+            with open(path, 'rb') as f:
+                metadata_str = f.read()
+            metadata_obj2 = Metadata.from_json(metadata_str)
 
-            # Assert that generic method instantiates the right inner class for
-            # each metadata type
+            # Assert that both methods instantiate the right inner class for
+            # each metadata type and ...
             self.assertTrue(
                     isinstance(metadata_obj.signed, inner_metadata_cls))
+            self.assertTrue(
+                    isinstance(metadata_obj2.signed, inner_metadata_cls))
+
+            # ... and return the same object (compared by dict representation)
+            self.assertDictEqual(
+                    metadata_obj.to_dict(), metadata_obj2.to_dict())
+
 
         # Assert that it chokes correctly on an unknown metadata type
         bad_metadata_path = 'bad-metadata.json'

tuf/api/metadata.py

@@ -29,7 +29,11 @@
 import tempfile
 
 from securesystemslib.formats import encode_canonical
-from securesystemslib.util import load_json_file, persist_temp_file
+from securesystemslib.util import (
+    load_json_file,
+    load_json_string,
+    persist_temp_file
+)
 from securesystemslib.storage import StorageBackendInterface
 from securesystemslib.keys import create_signature, verify_signature
 from tuf.repository_lib import (
@@ -80,6 +84,24 @@ def to_dict(self) -> JsonDict:
             'signed': self.signed.to_dict()
         }
 
+    @classmethod
+    def from_json(cls, metadata_json: str) -> 'Metadata':
+        """Loads JSON-formatted TUF metadata from a string.
+
+        Arguments:
+            metadata_json: TUF metadata in JSON-string representation.
+
+        Raises:
+            securesystemslib.exceptions.Error, ValueError, KeyError: The
+                metadata cannot be parsed.
+
+        Returns:
+            A TUF Metadata object.
+
+        """
+        return cls.from_dict(load_json_string(metadata_json))
+
+
     def to_json(self, compact: bool = False) -> None:
         """Returns the optionally compacted JSON representation of self. """
         return json.dumps(