Predicate object cannot be optional?

[mihaimaruseac]

Hello,

I think I may be holding this wrong. In https://github.com/in-toto/attestation/blob/main/spec/v1/predicate.md it is said that the predicate itself can be missing:

plus an optional predicate JSON object containing additional, type-dependent parameters.

However, the Python constructor requires the predicate to be set:

attestation/python/in_toto_attestation/v1/statement.py

Line 10 in fc43990

def __init__(self, subjects: list, predicate_type: str, predicate: dict) -> None:

For my (very test-y, WIP-y) use case I don't currently have anything to put in the predicate, so I pass {}.

However, when I then do statement.validate(), it fails to validate due to

attestation/python/in_toto_attestation/v1/statement.py

Lines 45 to 46 in fc43990

	if len(self.pb.predicate) == 0:
	raise ValueError("Predicate object required")

Should we always have something for the predicate? I was thinking that if I am able to record the subjects and don't have any additional metadata for my custom predicate I should be able to skip having to pass in a predicate object.

Happy to update documentation or code, depending on what is the desired path forward.