state_machine

state machine for humans

There are two types of developers in this world: those who love state machines and those who will eventually.

I fall in the first camp. I think it is really important to have a declarative way to define the states of an object. That’s why I developed state_machine.

Install

pip install state_machine

Basic Usage

@acts_as_state_machine
class Person():
    name = 'Billy'

    sleeping = State(initial=True)
    running = State()
    cleaning = State()

    run = Event(from_states=sleeping, to_state=running)
    cleanup = Event(from_states=running, to_state=cleaning)
    sleep = Event(from_states=(running, cleaning), to_state=sleeping)

    @before('sleep')
    def do_one_thing(self):
        print "{} is sleepy".format(self.name)

    @before('sleep')
    def do_another_thing(self):
        print "{} is REALLY sleepy".format(self.name)

    @after('sleep')
    def snore(self):
        print "Zzzzzzzzzzzz"

    @after('sleep')
    def big_snore(self):
        print "Zzzzzzzzzzzzzzzzzzzzzz"

person = Person()
print person.current_state == Person.sleeping       # True
print person.is_sleeping                            # True
print person.is_running                             # False
person.run()
print person.is_running                             # True
person.sleep()

# Billy is sleepy
# Billy is REALLY sleepy
# Zzzzzzzzzzzz
# Zzzzzzzzzzzzzzzzzzzzzz

print person.is_sleeping                            # True

Features

Before / After Callback Decorators

You can add callback hooks that get executed before or after an event (see example above).

Important: if the before event causes an exception or returns False, the state will not change (transition is blocked) and the after event will not be executed.

Blocks invalid state transitions

An InvalidStateTransition Exception will be thrown if you try to move into an invalid state.

ORM support

We have basic support for mongoengine, and sqlalchemy.

Mongoengine

Just have your object inherit from mongoengine.Document and state_machine will add a StringField for state.

Note: You must explicitly call #save to persist the document to the datastore.

@acts_as_state_machine
class Person(mongoengine.Document):
    name = mongoengine.StringField(default='Billy')

    sleeping = State(initial=True)
    running = State()
    cleaning = State()

    run = Event(from_states=sleeping, to_state=running)
    cleanup = Event(from_states=running, to_state=cleaning)
    sleep = Event(from_states=(running, cleaning), to_state=sleeping)

    @before('sleep')
    def do_one_thing(self):
        print "{} is sleepy".format(self.name)

    @before('sleep')
    def do_another_thing(self):
        print "{} is REALLY sleepy".format(self.name)

    @after('sleep')
    def snore(self):
        print "Zzzzzzzzzzzz"

    @after('sleep')
    def snore(self):
        print "Zzzzzzzzzzzzzzzzzzzzzz"


person = Person()
person.save()
eq_(person.current_state, Person.sleeping)
assert person.is_sleeping
assert not person.is_running
person.run()
assert person.is_running
person.sleep()
assert person.is_sleeping
person.run()
person.save()

person2 = Person.objects(id=person.id).first()
assert person2.is_running

Sqlalchemy

All you need to do is have sqlalchemy manage your object. For example:

from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
@acts_as_state_machine
class Puppy(Base):
   ...

Issues / Roadmap:

• Allow multiple state_machines per object
• Be able to configure the state field

Questions / Issues

Feel free to ping me on twitter: @tushman or add issues or PRs at https://github.com/jtushman/state_machine

Thank you

to aasm and ruby’s state_machine and all other state machines that I loved before