django-fsm adds declarative states management for django models. Instead of adding some state field to a django model, and manage it values by hand, you could use FSMState field and mark model 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.

$ pip install django-fsm

Or, for the latest git version

$ pip install -e git://github.com/kmmbvnr/django-fsm.git#egg=django-fsm

Library have full Python 3 support, for graph transition drawing you should install python3 compatible graphviz version from git+https://github.com/philipaxer/pygraphviz

Add FSMState field to your model

from django_fsm.db.fields import FSMField, transition

class BlogPost(models.Model):
    state = FSMField(default='new')

Use the transition decorator to annotate model methods

@transition(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, but not written to the database.

from django_fsm.db.fields 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 you are using the transition decorator with the save argument set to True, the new state will be written to the database

@transition(source='new', target='published', save=True)
def publish(self):
    """
    Side effects other than changing state goes here
    """

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 takes one argument, the model 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 model methods

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

Use the conditions like this:

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

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

You could instantiate field with protected=True option, that prevents direct state field modification

class BlogPost(models.Model):
    state = FSMField(default='new', protected=True)

model = BlogPost()
model.state = 'invalid' # Raises AttributeError

You could specify FSMField explicitly in transition decorator.

class BlogPost(models.Model):
    state = FSMField(default='new')

    @transition(field=state, source='new', target='published')
    def publish(self):
	    pass

This allows django_fsm to contribute to model class get_available_FIELD_transitions method, that returns list of (target_state, method) available from current model state

If you store the states in the db table you could use FSMKeyField to ensure Foreign Key database integrity.

You can also use FSMIntegerField. This is handy when you want to use enum style constants. This field is also db_index=True by default for speedy db loookups.

class BlogPostStateEnum(object):
    NEW = 10
    PUBLISHED = 20
    HIDDEN = 30

class BlogPostWithIntegerField(models.Model):
    state = FSMIntegerField(default=BlogPostStateEnum.NEW)

    @transition(source=BlogPostStateEnum.NEW, target=BlogPostStateEnum.PUBLISHED)
    def publish(self):
        pass

django_fsm.signals.pre_transition and django_fsm.signals.post_transition are called before and after allowed transition. No signals on invalid transition are called.

Arguments sent with these signals:

sender The model class.

instance The actual instance being procceed

name Transition name

source Source model state

target Target model state

Renders a graphical overview of your models states transitions

# Create a dot file
$ ./manage.py graph_transitions > transitions.dot

# Create a PNG image file only for specific model
$ ./manage.py graph_transitions -o blog_transitions.png myapp.Blog

django-fsm 1.5.1 2014-01-04

* Ad-hoc support for state fields from proxy and inherited models

django-fsm 1.5.0 2013-09-17

* Python 3 compatibility

django-fsm 1.4.0 2011-12-21

* Add graph_transition command for drawing state transition picture

django-fsm 1.3.0 2011-07-28

* Add direct field modification protection

django-fsm 1.2.0 2011-03-23

* Add pre_transition and post_transition signals

django-fsm 1.1.0 2011-02-22

* Add support for transition conditions 
* Allow multiple FSMField in one model
* Contribute get_available_FIELD_transitions for model class

django-fsm 1.0.0 2010-10-12

* Initial public release