feature: expect_override decorator to mark model methods safe for overriding

As of now, transition will not decorate models with trigger and convenience functions if they are already defined on the model. The check is done in Machine._checked_assignment. Users had to override _checked_assignment if they wanted to define model methods (e.g. for type checking) and have them overridden by transitions during runtime. This approach is an all or nothing solution. With expect_override, transitions now features an approach where developers can explicitly mark model methods that are intended to be overriden.

from transitions import Machine
from transitions.experimental.decoration import expect_override

class Model:

    @expect_override
    def is_A(self) -> bool:  # will be overridden during initialisation of Machine
        raise RuntimeError("Should be overridden")

    def is_B(self) -> bool:  # will NOT be overridden
        return False

    @expect_override
    def is_C(self) -> bool:  # will be overridden when state C is added later on
        return False

model = Model()

machine = Machine(model, states=["A", "B"], initial="A")

This is a step in the effort to improve the typing support of transitions.

Some related issues:

pytransitions / transitions

feature: expect_override decorator to mark model methods safe for overriding #676

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