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
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.
@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'.
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
TransitionNotAllowed exception is thrown.
Add a plain old python field to your POPO from popo_fsm import transition
class BlogPost(object): def __init__(): self.state = 'new'
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
conditions must be a list of functions
that take one argument, the POPO instance. The function must return either
False or a value that evaluates to
False. If all
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 """
How does popo-fsm diverge from sqlalchemy-fsm?
Works with POPOs, doesn't depend on sqlalchemy
Has no special support for sqlalchemy
Supports multiple state fields in a single object