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Plain old python object finite state machine support.

Project description

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.

@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.


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;

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

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 > 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

Project details

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