Metadata API: Metadata "factory" constructors misbehave with inheritance

class DerivedMetadata(Metadata):
    def ok(self):
        print("ok")

md = DerivedMetadata.from_file("root.json")
md.ok() # fails because md type is Metadata, not DerivedMetadata

This seems to happen because the construction path goes

• DerivedMetadata.from_file()
• DerivedMetadata.from_bytes()
• JsonDeserializer.deserialize() here deserializer doesn’t know the correct class anymore and calls Metadata.from_dict()
• Metadata.from_dict()

[lukpueh]

@jku, unless you need this resolved quickly, I think it would be a good self-contained task for someone to get started with python-tuf development. AFAICS what’s missing is:

• apply/revise patch from above
• adopt MetadataDeserializer accordingly (beware of type check case handling and cyclic-imports!)
• add a test

[jku]

so it should mean Metadata is a Generic, and it’s generic over the type T – this is often used to mean that the Generic is container for type T, as it is here (even if our container is a bit unique in that there is always one and only one T in Metadata).