Metadata API: add Metadata.to_bytes()

Metadata.to_bytes() is missing from the API and that is now becoming
annoying when writing the tests.
I think it makes sense to add:
it'll complete the serializing counterparts to from_bytes()/from_file().

We can also reuse to_bytes() in to_file() and that way ensure we don't
import the JSONSerializer locally twice.

Signed-off-by: Martin Vrachev <[email protected]>

Author: Martin Vrachev

tests/test_api.py

@@ -159,6 +159,28 @@ def test_read_write_read_compare(self):
             os.remove(path_2)
 
 
+    def test_to_from_bytes(self):
+        for metadata in ["root", "snapshot", "timestamp", "targets"]:
+            path = os.path.join(self.repo_dir, 'metadata', metadata + '.json')
+            with open(path, 'rb') as f:
+                metadata_bytes = f.read()
+            metadata_obj = Metadata.from_bytes(metadata_bytes)
+            # Comparate that from_bytes/to_bytes doesn't change the content
+            # for two cases for the serializer: noncompact and compact.
+
+            # Case 1: test noncompact by overriding the default serializer.
+            self.assertEqual(
+                metadata_obj.to_bytes(JSONSerializer()), metadata_bytes
+            )
+
+            # Case 2: test compact by using the default serializer.
+            obj_bytes = metadata_obj.to_bytes()
+            metadata_obj_2 = Metadata.from_bytes(obj_bytes)
+            self.assertEqual(
+                metadata_obj_2.to_bytes(), obj_bytes
+            )
+
+
     def test_sign_verify(self):
         root_path = os.path.join(self.repo_dir, 'metadata', 'root.json')
         root:Root = Metadata.from_file(root_path).signed

tuf/api/metadata.py

@@ -185,6 +185,29 @@ def from_bytes(
 
         return deserializer.deserialize(data)
 
+    def to_bytes(
+        self, serializer: Optional[MetadataSerializer] = None
+    ) -> bytes:
+        """Return the serialized TUF file format as bytes.
+
+        Arguments:
+            serializer: A MetadataSerializer instance that implements the
+                desired serialization format. Default is JSONSerializer.
+
+        Raises:
+            tuf.api.serialization.SerializationError:
+                The metadata object cannot be serialized.
+        """
+
+        if serializer is None:
+            # Use local scope import to avoid circular import errors
+            # pylint: disable=import-outside-toplevel
+            from tuf.api.serialization.json import JSONSerializer
+
+            serializer = JSONSerializer(compact=True)
+
+        return serializer.serialize(self)
+
     def to_dict(self) -> Dict[str, Any]:
         """Returns the dict representation of self."""
 
@@ -214,15 +237,10 @@ def to_file(
                 The file cannot be written.
         """
 
-        if serializer is None:
-            # Use local scope import to avoid circular import errors
-            # pylint: disable=import-outside-toplevel
-            from tuf.api.serialization.json import JSONSerializer
-
-            serializer = JSONSerializer(compact=True)
+        bytes_data = self.to_bytes(serializer)
 
         with tempfile.TemporaryFile() as temp_file:
-            temp_file.write(serializer.serialize(self))
+            temp_file.write(bytes_data)
             persist_temp_file(temp_file, filename, storage_backend)
 
     # Signatures.