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State Transition System for Django

Project description

# State Transition System (STS) for Django

[State Transition Systems][1] 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.


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