> exec() params are already checked, as a seconds param, only dict or dict subclasses are accepted. Seems like good enough contract.

As I said, the documentation is considered the public API contract and again, the documentation says that dict subclasses are not accepted:

>> it must be a dictionary (and not a subclass of dictionary)

My opinion is that as Terry said, this is a violation of the contract and supporting dict subclasses is a case unusual enough to not sacrifice performance for the general case. 

> I am not, and please stop your accusations, that violates CoC.

A respectful indication to keep the discussion civilized does not violate the CoC. If you are not being sarcastic, then everything is fine.