Finite state machine field for plain old python objects (POPOs) (based on sqlalchemy-fsm)

popo-fsm adds declarative states management for plain old python objects (POPO). Instead of adding some state field to a POPO, and managing its values by hand, you could use a plain old python field and mark POPO methods with the transition decorator. Your method will contain the side-effects of the state change.

The decorator also takes a list of conditions, all of which must be met before a transition is allowed.

Definition

@transition('field_name', source='state1', target='state2')

Here, 'field_name' is the name of the field/property/attribute which needs to change from 'state1' to 'state2'.

The source can also be a list of possible states/values from which the field can move to the target state/value. e.g.

@transition('field_name', source=['new', 'draft'], target='published')

In case the decorated method is called when the field value is not one of those specified in source, a TransitionNotAllowed exception is thrown.

Example

Add a plain old python field to your POPO from popo_fsm import transition

class BlogPost(object):
    def __init__():
        self.state = 'new'

Use the transition decorator to annotate POPO methods

@transition('state', source='new', target='published')
def publish(self):
    """
    This function may contain side-effects, 
    like updating caches, notifying users, etc.
    The return value will be discarded.
    """

source parameter accepts a list of states, or an individual state. You can use * for source, to allow switching to target from any state.

If calling publish() succeeds without raising an exception, the state field will be changed.

from popo_fsm import can_proceed

def publish_view(request, post_id):
    post = get_object__or_404(BlogPost, pk=post_id)
    if not can_proceed(post.publish):
         raise Http404;

    post.publish()
    post.save()
    return redirect('/')

If your given function requires arguments to validate, you need to include them when calling can_proceed as well as including them when you call the function normally. Say publish() required a date for some reason:

if not can_proceed(post.publish, the_date):
    raise Http404
else:
    post.publish(the_date)

If you require some conditions to be met before changing state, use the conditions argument to transition. conditions must be a list of functions that take one argument, the POPO instance. The function must return either True or False or a value that evaluates to True or False. If all functions return True, all conditions are considered to be met and transition is allowed to happen. If one of the functions return False, the transition will not happen. These functions should not have any side effects.

You can use ordinary functions

def can_publish(instance):
    # No publishing after 17 hours
    if datetime.datetime.now().hour > 17:
       return False
    return True

Or POPO methods

def can_destroy(self):
    return self.is_under_investigation()

Use the conditions like this:

@transition('state', source='new', target='published', conditions=[can_publish])
def publish(self):
    """
    Side effects galore
    """

@transition('state', source='*', target='destroyed', conditions=[can_destroy])
def destroy(self):
    """
    Side effects galore
    """