First result

A spec marked firstresult stops at the first implementation that returns a non-None value and returns that value directly - not a list. It is the right shape for selection and resolution: cup choosers, routers, resolvers.

@extension_point(firstresult=True)
def choose_cup(size: str) -> str | None:
    """Pick a cup for the size; the first plugin to answer wins."""

Abstaining

An implementation opts out of a particular call by returning None. The next implementation in order is then given a chance.

class SmallCupPlugin:
    @extension
    def choose_cup(self, size: str) -> str | None:
        return "8oz paper cup" if size == "small" else None   # abstain otherwise

A fallback that always answers

Mark a catch-all trylast so the specific plugins are consulted first and the fallback only runs when they all abstain:

class FallbackCupPlugin:
    @extension(trylast=True)
    def choose_cup(self, size: str) -> str | None:
        return "16oz default cup"

Run it

make run DEMO=firstresult

 small -> 8oz paper cup
 large -> 20oz tumbler
medium -> 16oz default cup

Notes

• If every implementation abstains, the call returns None.
• firstresult cannot be combined with historic; the manager rejects that at add_extension_points time.

See also the notification router example, which routes a message to the first channel that accepts it.