Move metadata class model de/serialization to sub-package

tox.ini

@@ -42,6 +42,9 @@ commands =
 
 [testenv:lint]
 commands =
-    pylint {toxinidir}/tuf --ignore={toxinidir}/tuf/api
+    # Use different pylint configs for legacy and new (tuf/api) code
+    # NOTE: Contrary to what the pylint docs suggest, ignoring full paths does
+    # work, unfortunately each subdirectory has to be ignored explicitly.
+    pylint {toxinidir}/tuf --ignore={toxinidir}/tuf/api,{toxinidir}/tuf/api/serialization
     pylint {toxinidir}/tuf/api --rcfile={toxinidir}/tuf/api/pylintrc
     bandit -r {toxinidir}/tuf

tuf/api/pylintrc

@@ -4,3 +4,6 @@ disable=fixme
 [FORMAT]
 indent-string="    "
 max-line-length=79
+
+[DESIGN]
+min-public-methods=0

tuf/api/serialization/__init__.py

@@ -0,0 +1,43 @@
+"""TUF role metadata de/serialization.
+
+This sub-package provides abstract base classes and concrete implementations to
+serialize and deserialize TUF role metadata and metadata parts.
+
+Any custom de/serialization implementations should inherit from the abstract
+base classes defined in this __init__.py module.
+
+- Metadata de/serializers are used to convert to and from wireline formats.
+- Signed serializers are used to canonicalize data for cryptographic signatures
+  generation and verification.
+
+"""
+import abc
+
+class MetadataDeserializer():
+    """Abstract base class for deserialization of Metadata objects. """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def deserialize(self, raw_data: bytes) -> "Metadata":
+        """Deserialize passed bytes to Metadata object. """
+        raise NotImplementedError
+
+
+class MetadataSerializer():
+    """Abstract base class for serialization of Metadata objects. """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def serialize(self, metadata_obj: "Metadata") -> bytes:
+        """Serialize passed Metadata object to bytes. """
+        raise NotImplementedError
+
+
+class SignedSerializer():
+    """Abstract base class for serialization of Signed objects. """
+    __metaclass__ = abc.ABCMeta
+
+    @abc.abstractmethod
+    def serialize(self, signed_obj: "Signed") -> bytes:
+        """Serialize passed Signed object to bytes. """
+        raise NotImplementedError