Remove Signed.read_from_json metadata method

Remove metadata factory on Signed class, for the sake of API
simplicity/non-ambiguity, i.e. it's enough to have one
way of loading any Metadata, that is:
Metadata.read_from_json

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

tests/test_api.py

@@ -91,15 +91,10 @@ def test_generic_read(self):
             path = os.path.join(self.repo_dir, 'metadata', metadata + '.json')
             metadata_obj = Metadata.read_from_json(path)
 
-            # Assert that generic method ...
-            # ... instantiates the right inner class for each metadata type
+            # Assert that generic method instantiates the right inner class for
+            # each metadata type
             self.assertTrue(
                     isinstance(metadata_obj.signed, inner_metadata_cls))
-            # ... and reads the same metadata file as the corresponding method
-            # on the inner class would do (compare their dict representation)
-            self.assertDictEqual(
-                    metadata_obj.as_dict(),
-                    inner_metadata_cls.read_from_json(path).as_dict())
 
         # Assert that it chokes correctly on an unknown metadata type
         bad_metadata_path = 'bad-metadata.json'
@@ -185,7 +180,7 @@ def test_metadata_base(self):
         # with real data
         snapshot_path = os.path.join(
                 self.repo_dir, 'metadata', 'snapshot.json')
-        md = Snapshot.read_from_json(snapshot_path)
+        md = Metadata.read_from_json(snapshot_path)
 
         self.assertEqual(md.signed.version, 1)
         md.signed.bump_version()
@@ -200,7 +195,7 @@ def test_metadata_base(self):
     def test_metadata_snapshot(self):
         snapshot_path = os.path.join(
                 self.repo_dir, 'metadata', 'snapshot.json')
-        snapshot = Snapshot.read_from_json(snapshot_path)
+        snapshot = Metadata.read_from_json(snapshot_path)
 
         # Create a dict representing what we expect the updated data to be
         fileinfo = snapshot.signed.meta
@@ -216,7 +211,7 @@ def test_metadata_snapshot(self):
     def test_metadata_timestamp(self):
         timestamp_path = os.path.join(
                 self.repo_dir, 'metadata', 'timestamp.json')
-        timestamp = Timestamp.read_from_json(timestamp_path)
+        timestamp = Metadata.read_from_json(timestamp_path)
 
         self.assertEqual(timestamp.signed.version, 1)
         timestamp.signed.bump_version()

tuf/api/metadata.py

@@ -32,9 +32,6 @@
 from securesystemslib.util import load_json_file, persist_temp_file
 from securesystemslib.storage import StorageBackendInterface
 from securesystemslib.keys import create_signature, verify_signature
-from tuf.repository_lib import (
-    _strip_version_number
-)
 
 import iso8601
 import tuf.formats
@@ -288,46 +285,6 @@ def as_dict(self) -> JsonDict:
             'expires': self.expires
         }
 
-    @classmethod
-    def read_from_json(
-            cls, filename: str,
-            storage_backend: Optional[StorageBackendInterface] = None
-            ) -> Metadata:
-        signable = load_json_file(filename, storage_backend)
-        """Loads corresponding JSON-formatted metadata from file storage.
-
-        Arguments:
-            filename: The path to read the file from.
-            storage_backend: An object that implements
-                securesystemslib.storage.StorageBackendInterface. Per default
-                a (local) FilesystemBackend is used.
-
-        Raises:
-            securesystemslib.exceptions.StorageError: The file cannot be read.
-            securesystemslib.exceptions.Error, ValueError: The metadata cannot
-                be parsed.
-
-        Returns:
-            A TUF Metadata object whose signed attribute contains an object
-            of this class.
-
-        """
-        # FIXME: It feels dirty to access signable["signed"]["version"] here in
-        # order to do this check, and also a bit random (there are likely other
-        # things to check), but later we don't have the filename anymore. If we
-        # want to stick to the check, which seems reasonable, we should maybe
-        # think of a better place.
-        _, fn_prefix = _strip_version_number(filename, True)
-        if fn_prefix and fn_prefix != signable['signed']['version']:
-            raise ValueError(
-                f'version filename prefix ({fn_prefix}) must align with '
-                f'version in metadata ({signable["signed"]["version"]}).')
-
-        return Metadata(
-            signed=cls(**signable['signed']),
-            signatures=signable['signatures'])
-
-
 class Timestamp(Signed):
     """A container for the signed part of timestamp metadata.