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Fluidity: state machine implementation for Python objects

Project description

State machine implementation for Python objects.

How to use

A very simple example taken from specs:

from fluidity import StateMachine, state, transition

class SimpleMachine(StateMachine):

     initial_state = 'created'


     transition(from_='created', event='queue', to='waiting')
     transition(from_='waiting', event='process', to='processed')
     transition(from_=['waiting', 'created'], event='cancel', to='canceled')

“A slightly more complex example”

For demonstrating more advanced capabilities, a “slightly more complex example” from AASM, the Ruby’s most popular state machine implementation, is reproduced below, using Fluidity:

from fluidity import StateMachine, state, transition

class Relationship(StateMachine):
    initial_state = lambda relationship: relationship.strictly_for_fun() and 'intimate' or 'dating'
    state('dating', enter='make_happy', exit='make_depressed')
    state('intimate', enter='make_very_happy', exit='never_speak_again')
    state('married', enter='give_up_intimacy', exit='buy_exotic_car_and_buy_a_combover')

    transition(from_='dating', event='get_intimate', to='intimate', guard='drunk')
    transition(from_=['dating', 'intimate'], event='get_married', to='married', guard='willing_to_give_up_manhood')

    def strictly_for_fun(self): pass
    def drunk(self): pass
    def willing_to_give_up_manhood(self): return True
    def make_happy(self): pass
    def make_depressed(self): pass
    def make_very_happy(self): pass
    def never_speak_again(self): pass
    def give_up_intimacy(self): pass
    def buy_exotic_car_and_buy_a_combover(self): pass


A Fluidity state machine must have one initial state and at least two states.

A state may have enter and exit callbacks, for running some code on state enter and exit, respectively. These params can be method names (as strings), callables, or lists of method names or callables.


Transitions lead the machine from a state to another. Transitions must have from_, to, and action parameters. from_ is one or more (as list) states from which the transition can be started. to is the state to which the transition will lead the machine. event is the method that have to be called to launch the transition. This method is automatically created by the Fluidity engine.

A transition can have optional action and guard parameters. action is a method (or callable) that will be called when transition is launched. If parameters are passed to the event method, they are passed to the action method, if it accepts these parameters. guard is a method (or callable) that is called to allow or deny the transition, depending on the result of its execution. Both “action” and guard can be lists.

The same event can be in multiple transitions, going to different states, having their respective guards as selectors. For the transitions having the same event, only one guard should return a true value at a time.


States and transitions are defined in a class-wide mode. However, one can define states and transitions for individual objects. For example, having “door” as a state machine:

door.add_transition(from_='closed', event='crack', to='broken')

These additions only affect the target object.

How to install

Just run:

pip install fluidity-sm

Note: the Pypi package is called fluidity-sm, not fluidity.

How to run tests

Just run:

make test

for install all test dependencies (Should-DSL and Specloud, at the moment) and run the tests. Fluidity itself has no dependencies.

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