State Transition System (STS) for Django
State Transition Systems have less constraints than Finite State Automata, and can be utilized for various use cases.
The core components include:
- State
- Event
- Transition
- System
Events cause a transition from some state to a new state within a given system.
The API supports defining immediate transitions and long-running transitions. Now, for a riveting example..
system = System(name='Example 1')
system.save()
# Immediate transition.. event => state
system.transition('Door Opened', event='Open Door')
# 'Long-running' transitions.. event happens
system.start_transition('Close Door Slowly')
# Time passes..
time.sleep(2)
# The resulting state..
system.end_transition('Door Closed')To remove the boilerplate from the above example, use the transition
context manager instead:
from sts.contextmanagers import transition
with transition('Example 1', 'Door Closed', event='Close Door Slowly'):
time.sleep(2)A model object can be associated directly with a System using Django's
ContentTypes framework generic foreign keys.
door = Door.objects.get(name='Door #1')
system = System(content_object=door)
# ...System objects have a few extra conveniences:
# number of transitions
len(system) == system.length
# iteration starting with the first transition
for trans in system:
...
# indexing and slices
system[:3] # first 3 transitions
system[-3:] # last 3 transitions
system[:-3] # all except the last 3 transitions
system[1:3] # arbitrary slice
system[2] # specific transitionThis enables bringing in django-sts to an existing model to begin tracking states of objects.
It even comes with an abstract STSModel that augments a model with the above
methods for seamless integration (it does not add any model fields):
class Door(STSModel):
name = models.CharField(max_length=20)
door = Door()
door.save()
door.transition('Door Closed', event='Close Door')The library leaves it up to the application to implement the constraints of a finite state automata/machine.
