How to restart a state machine

[schaefer01]

Hi, At this point in the game I'm assuming that there is an undocumented step that I'm not doing
that makes the example work.

Maybe a function named "change_shape" ?

If so, then what does change_shape() look like?

This is the section titled: "Reflexive transitions from multiple states"

The tutorial states (pun intended):

A reflexive trigger (trigger that has the same state as source and destination) can easily be added specifying = as destination. This is handy if the same reflexive trigger should be added to multiple states. For example:

machine.add_transition('touch', ['liquid', 'gas', 'plasma'], '=', after='change_shape')
This will add reflexive transitions for all three states with touch() as trigger and with change_shape executed after each trigger.

My code to demonstrate:

class Matter(object):
pass

lump = Matter()

from transitions import Machine

machine = Machine(model=lump, states=['solid', 'liquid', 'gas', 'plasma'], initial='solid')

lump.state

states=['solid', 'liquid', 'gas', 'plasma']

transitions = [
{ 'trigger': 'melt', 'source': 'solid', 'dest': 'liquid' },
{ 'trigger': 'evaporate', 'source': 'liquid', 'dest': 'gas' },
{ 'trigger': 'sublimate', 'source': 'solid', 'dest': 'gas' },
{ 'trigger': 'ionize', 'source': 'gas', 'dest': 'plasma' }
]

machine = Machine(lump, states=states, transitions=transitions, initial='liquid')

lump.state
-> 'liquid'

machine.add_transition('touch', ['liquid', 'gas', 'plasma'], '=', after='change_shape')

lump.touch()
->
Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1210, in resolve_callable
func = getattr(event_data.model, func)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Matter' object has no attribute 'change_shape'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1217, in resolve_callable
module_name, func_name = func.rsplit('.', 1)
^^^^^^^^^^^^^^^^^^^^^^
ValueError: not enough values to unpack (expected 2, got 1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "", line 1, in
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 406, in trigger
return self.machine._process(func)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1240, in _process
return trigger()
^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 420, in _trigger
self._process(event_data)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 443, in _process
if trans.execute(event_data):
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 280, in execute
event_data.machine.callbacks(itertools.chain(self.after, event_data.machine.after_state_change), event_data)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1175, in callbacks
self.callback(func, event_data)
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1192, in callback
func = self.resolve_callable(func, event_data)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/transitions/core.py", line 1223, in resolve_callable
raise AttributeError("Callable with name '%s' could neither be retrieved from the passed "
AttributeError: Callable with name 'change_shape' could neither be retrieved from the passed model nor imported from a module.

[aleneum]

Hello @schaefer01,

change_shape is just a placeholder for a callback defined on you stateful object. You can replace it with any defined callback method on your model or with a (list of) references to it. I'll mention that in the README.

class Matter(object):
    
    def change_shape(self):
        print(f"I am {self.state} now")

lump = Matter()

machine = Machine(
    model=lump, states=["solid", "liquid", "gas", "plasma"], initial="liquid"
)
machine.add_transition('touch', ['liquid', 'gas', 'plasma'], '=', after='change_shape')

assert lump.touch()  # I am liquid now

[schaefer01]

Thank you. I wish the examples were more fully worked out.

How I learn is by trying the code as-is, and then I add variations and retry.

With respect to retrying, I'd like to know (ahead of time) and re: to your reply in issue #718,

what the non-idempotent configuration statements are.

Many of the pragmatics of programming that controls hardware is safe restarting on faults,

and this means knowing what a clean restart looks like.

I know this is my problem, not yours.

[aleneum]

If you split

• the model (e.g. Matter) with callbacks and (at least) one stateful property^1, and
• the machine (e.g. GraphMachine) with state and transition definitions

you can picture machine as a rulebook that is (or should be) stateless. A shutdown usually means to store the state property (and maybe other stateful properties). To load the model again, one would load the saved state, initialize the model and pass it to the Machine constructor with initial=last_known_state. Many transitions user load their machine configurations from yaml files. See the FAQ notebook for an example about how to do this.

^1: (in most cases the stateful model property is called 'state' which can be renamed in cases where 'state' must be reserved for other purposes on the stateful object; think django)

[schaefer01]

This makes sense but my use case is different where: 1. the statemachine is suspect and 2. user process is suspect, and 3. the hardware running the statemachine is suspect.

I could add a safe state but for debug and development before there is a well-tested safe state, I'd like a clean spot to restart the s/w development. I could always exit python and re-enter but it would be nice to restart the application from inside the application. If the statemachine itself fights the restart of the statemachine, this is a problem for me to do my work.
Isolating bugs steps:

1. restart the application inside the application
2. restart the application from restarting python
3. restart the application from restarting the account
4. restart the application from restarting the computer

This is like a database when you command it to drop tables during development before deployment, and the database replies "no I won't"

I'm thinking this is not the statemachine for me.

[aleneum]

Hello @schaefer01,

I could always exit python and re-enter but it would be nice to restart the application from inside the application.

I don't see where this is limited by how transition works

This is like a database when you command it to drop tables during development before deployment, and the database replies "no I won't"

I don't fully understand how this database analogy relates to stateful objects in OOP but as far as I understand you want to either a) have a clean sheet where re-initializing a stateful object would be the way to go in my opinion basically as shown in the quickstart:

from transitions import Machine

class MyStatefulObject:

    def __init__(self, states, transitions, initial) -> None:
        self.machine = Machine(self, states=states, transitions=transitions, initial=initial)
    
    def a_callback(self) -> None:
        pass # do something

    def a_condition_check(self) -> bool:
        return True
    
states = ["A", "B"]
x = MyStatefulObject(states, [["foo", "A", "B"]], "A")
x.foo()
x = MyStatefulObject(states, [["bar", "B", "A"]], x.state)
x.bar()
assert not hasattr(x, "foo")

or to b) reinitialize certain 'parts' of an already initialized stateful object. This has some drawbacks because transitions has no concept of what needs to be 'reset' on your model. However, as states in the FAQ under Transitions does not add convencience methods to my model:

There is a high chance that your model already contained a trigger method or methods with the same name as your even trigger. In this case, transitions will not add convenience methods to not accidentaly break your model and only emit a warning. If you defined these methods on purpose and want them to be overrided or maybe even call both -- the trigger event AND your predefined method, you can extend/override Machine._checked_assignment which is always called when something needs to be added to a model.

The override is pretty simple:

from transitions import Machine

class OverrideMachine(Machine):

    def _checked_assignment(self, model, name, func):
        setattr(model, name, func)


class Model:

    def a_callback(self) -> None:
        pass # do something

    def a_condition_check(self) -> bool:
        return True

states = ["A", "B"]
model = Model()
machine = OverrideMachine(model, states=states, transitions=[["foo", "A", "B"]], initial="A")  # [1]
model.foo()
assert model.is_B()
machine = OverrideMachine(model, states=states, transitions=[["foo", "B", "A"], ["bar", "A", "B"]], initial=model.state)  # [2]
model.foo()
assert model.is_A()
model.trigger("bar")
assert model.is_B()

OverrideMachine will ignore present object properties. But as I mentioned earlier this can be tricky since a newly constructed machine has no concept of what other machines (or state transitions) might have added to the model. This means that when OverrideMachine 1 adds triggers (e.g. "event_A") to the model it will stay assigned to model even after OverrideMachine 2 decorated model. This can cause memory leaks as references to the first machine may not be released properly.

If model bindings are more than you actually need and you don't mind some more verbose code, you can skip passing models to Machine and call triggers directly.

from transitions import Machine

class Model:

    def __init__(self):
        self.state = "A"

    def my_callback(self):
        print("wee")

model = Model()
machine = Machine(model=None, states=["A", "B"], transitions=[["foo", "A", "B"]])
machine.add_transition("bar", "B", "A")
machine.on_enter_B("my_callback")
assert model.state == "A"
machine.events["foo"].trigger(model)  # >>> wee
assert model.state == "B"
machine.events["bar"].trigger(model)  # >>> wee
assert model.state == "A"
machine = Machine(model=None, states=["A", "C"], transitions=[["baz", "*", "X"]], initial="X")
assert model.state == "A"
machine.events["baz"].trigger(model)  # >>> [no wee]
assert model.state == "X"

There is also some Typing Support to enable better type hints and checks but this does not really change how transitions operates.