Django States

Description

State engine for django models. Define a state graph for a model and remember the state of each object. State transitions can be logged for objects.

Installation

pip install django-states

Usage example

To use a state machine, you should add a state field to the model

from django_states.fields import StateField
from django_states.machine import StateMachine, StateDefinition, StateTransition

class PurchaseStateMachine(StateMachine):
   log_transitions = True

   # possible states
   class initiated(StateDefinition):
       description = _('Purchase initiated')
       initial = True

       def handler(self, instance):
           # To execute code in this handler make sure to save the instance with
           # obj.save(no_state_validation=False) on object creation
           code_to_execute_when_arriving_in_this_state()

   class paid(StateDefinition):
       description = _('Purchase paid')

       def handler(self, instance):
           code_to_execute_when_arriving_in_this_state()

   class shipped(StateDefinition):
       description = _('Purchase shipped')

   # state transitions
   class mark_paid(StateTransition):
       from_state = 'initiated'
       to_state = 'paid'
       description = 'Mark this purchase as paid'

   class ship(StateTransition):
       from_state = 'paid'
       to_state = 'shipped'
       description = 'Ship purchase'

       def handler(transition, instance, user, **kwargs):
           code_to_execute_during_this_transition()

       def has_permission(transition, instance, user):
           return true_when_user_can_make_this_transition()

class Purchase(StateModel):
    purchase_state = StateField(machine=PurchaseStateMachine, default='initiated')
    ... (other fields for a purchase)

If log_transitions is enabled, another model is created. Everything should be compatible with South_ for migrations.

Usage example:

p = Purchase()

# Will automatically create state object for this purchase, in the
# initial state.
p.save()
p.get_purchase_state_info().make_transition('mark_paid', request.user) # User parameter is optional
p.state # Will return 'paid'
p.get_purchase_state_info().description # Will return 'Purchase paid'

# Returns an iterator of possible transitions for this purchase.
p.get_purchase_state_info().possible_transitions

# Which can be used like this..
[x.get_name() for x in p.possible_transitions]

For better transition control, override:

• has_permission(self, instance, user): Check whether this user is allowed to make this transition. See: Validation
• validate(self, instance): Check whether this transition is allowed. See: Validation
• handler(self, instance, user, **kwargs): Code to run during this transition. When an exception has been raised in here, the transition will not be made. See: Handlers

Get all objects in a certain state:

Purchase.objects.filter(state='initiated')

Validation

You can add a test that needs to pass before a state transition can be executed. Well, you can add 2: one based on the current user (has_permission) and one generic (validate).

So on a StateTransition-object you need to specify an extra validate function (signature is validate(cls, instance)). This should yield TransitionValidationError, this way you can return multiple errors on that need to pass before the transition can happen.

The has_permission function (signature has_permission(transition, instance, user)) should check whether the given user is allowed to make the transition. E.g. a super user can moderate all comments while other users can only moderate comments on their blog-posts.

Handlers

You can execute code upon doing a state transition or when reaching a certain state.

For executing code when reaching a certain state add a handler function (signature is handler(cls, instance)) to your StateDefinition. When this state is reached successfully the code in the handler is executed. To execute the code in the handler of the initial state (StateDefinition with initial = True) make sure that you save the created object using obj.save(no_state_validation=False) (no_state_validation defaults to True).

To execute code when doing a certain state transition add a handler function (signature is handler(cls, instance, user, **kwargs)) to your StateTransition. This code is only executed when the state transition is allowed.

Example:

class ModerationMachine(StateMachine):
   """A simple state machine for moderating articles"""

   # States
   class pending(StateDefinition):
      description = 'Pending review'
      initial = True

      def handler(cls, instance):
         # When a new article is added a manager should review it, so email
         # all managers to notify them about the new article
         mail_managers(
            'New article pending review: {}'.format(instance.title),
            'Empty message')

   class approved(StateDefinition):
      description = 'Approved'

      def handler(cls, instance):
         send_mail(
            'Your article has been approved',
            'Empty message',
            '[email protected]',
            [instance.author.email])

   class rejected(StateDefinition):
      description = 'Rejected'

   # State transitions
   class reject(StateTransition):
      from_states = ['pending']
      to_state = 'rejected'
      description = 'Reject submitted article'

      def handler(cls, instance, user, reason):
         # The handler in the StateTransition can also be used,
         # and can receive additional **kwargs from a state transition
         # in this case to relay a reason for rejection
         send_mail(
            'Your article has been rejected',
            'Reason for rejection: {}'.format(reason),
            '[email protected]',
            [instance.author.email])

   class approve(StateTransition):
      from_states = ['pending']
      to_state = 'approved'
      description = 'Approve submitted article'


class Article(models.Model):
   title = models.CharField(max_length=100)
   author = models.ForeignKey(User)

   state = StateField(machine=ModerationMachine)


# Example code:
author = User.objects.create_user('Erik', '[email protected]', 'mypassword')
article = Article(title='Using migrations in Django', author=author)
article.save(no_state_validation=False)
# Email is send to all managers notifying them of a new article pending review
article.get_state_info().make_transition('reject', reason='Improper writing style')
# Email is send to the author detailing the reason for rejection

Note

If you're creating a DataMigration in South, remember to use obj.save(no_state_validation=True)

Groups

Sometimes you want to group several states together, since for a certain view (or other content) it doesn't really matter which of the states it is. We support 2 different state groups, inclusive (only these) or exclusive (everything but these):

class is_paid(StateGroup):
    states = ['paid', 'shipped']

class is_paid(StateGroup):
    exclude_states = ['initiated']

State graph

You can get a graph of your states by running the graph_states management command.

python manage.py graph_states myapp.Purchase.state

This requires graphviz and python bindings for graphviz: pygraphviz and yapgvb.