State Transition System for Django
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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')

# Immediate transition.. event => state
system.transition('Door Opened', event='Open Door')

# 'Long-running' transitions.. event happens
system.start_transition('Close Door Slowly')

# Time passes..

# 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'):

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 transition

This 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.transition('Door Closed', event='Close Door')

The library leaves it up to the application to implement the constraints of a finite state automata/machine.