PEP 544 includes a Support optional protocol members section which says:
In the interest of simplicity, we propose to not support optional methods or attributes. We can always revisit this later if there is an actual need.
I wanted to call out an ‘actual need’ here in the falcon package (which is in the process of adding typing).
It has a duck typed ‘resource’ which provides optional methods for the HTTP verbs (on_get, on_post, etc.). Accordingly it has exactly the semantics given in the PEP:
a method or data attribute does not need to be present in a class implementing a protocol, but if it is present, it must conform to a specific signature or type.
Perhaps we can collect other such use cases here as a motivation to implement optional protocol members in the future.