Approach to implementing the State Pattern

[johnesher]

Hi,
I'm evaluating state machines for a project controlling some mechanics.
I wish to use the State Pattern, where each state is represented as a class instance which encapsulates all of the actions that can happen within that state.

I have used a different package in the past but it was not well matched to the State Pattern and hence am looking at alternatives.

Many of my states will have multiple methods to handle different Events rather than simple on_enter(...) and on_exit(...) methods, and there are likely to be some 20 states, possibly in a simple hierarchy.

Presumably the approach would be to create sub-classes of the State class.

Can anybody point me towards an example of this approach or comment on how easy or difficult it would be to implement?

[fgmacedo]

Hi @johnesher , how are you? This state pattern is not directly applied in a statemachine library like python-statemachine.

Are the actions of the same event on distinct states the same, or they can differ?

If they stay the same, you can easily use the statemachine approach mapping events to actions, and events to all possible transitions.

[johnesher]

Thanks @fgmacedo,

No, the actions of an event will often differ, for example, an event might be received in a state when that event is not expected, or an event "stop" would do something very different in a state "heating" compared to a state "opening-door".

To implement a State Pattern in python-statemachine I would expect to add the actions to a State class rather than to a StateMachine class.

I'm aiming for an architecture where the concerns of the state machine engine are separated from the concerns of actions and it is clear which actions are available in each state.

[fgmacedo]

Can you elaborate a concrete example?