[Feature] [Request] Implement State History

[doanguyen]

As the title, it would be super helpful but I have not come up with a feasible idea yet, but people in event sourcing has done it before. Do you have time to implement or discuss it?

Btw, thanks a lot for a very good job!

[aleneum]

Hi @doanguyen,

what are you looking for? Just a list of state names? Since the stateful object with transitions is the model you can use the state property to track state changes:

from transitions import Machine
import collections


# if you like your machine to act as a model, let Model inherit from Machine
class Model(object):

    def __init__(self, history_length):
        self.state_history = collections.deque(maxlen=history_length)

    @property
    def state(self):
        return self.state_history[-1]

    @state.setter
    def state(self, value):
        self.state_history.append(value)


model = Model(3)
machine = Machine(model, states=['A', 'B', 'C', 'D'], initial='A')
print(model.state)  # >>> A

model.to_B()
model.to_C()
model.to_A()
model.to_D()

print("->".join(model.state_history))  # >>> C->A->D

[doanguyen]

Hi,
I'm actually looking for a way of storing states and its transitions into database and re-initialize later.
After a while, I think storing events with triggered time could work as well.

[aleneum]

I'm actually looking for a way of storing states and its transitions into database and re-initialize later

States are persistent objects maintained by a Machine. States can be pickled (serialized) and deserialized if necessary. However, the State objects themselves do not (or should not) contain actual state related properties. They are rather 'blueprints' to determine what should happen with a 'stateful' object which is a model.

You might want to have a look at this issue or this notebook to get an idea how models and database table schemes can be connected.
This way you can save and load complete model/object states.

[doanguyen]

Thanks a bunch! I've got it.