django-state-manager is based on django-fsm by Mikhail Podgurskiy and provides implementation of states and transitions for Django models.
Additionally, it provides the coditions framework for implementing contitions for actions in an easy declarative way.
pip install django-state-managerAdd FSMState field to your model
from django_state_manager.fsm import FSMField, transition
class BlogPost(models.Model):
state = FSMField(default='new')Use the transition decorator to annotate model methods
@transition(field=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.
"""
The field parameter accepts both a string attribute name or an
actual field instance.
If calling publish() succeeds without raising an exception, the state
field will be changed, but not written to the database.
from django_state_manager.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 PermissionDenied
post.publish()
post.save()
return redirect('/')
If some conditions are required to be met before changing the state, use
the conditions argument to transition. conditions must be a
list of functions taking 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 the transition is allowed to happen. If one
of the functions returns 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(field=state, source='new', target='published', conditions=[can_publish])
def publish(self):
"""
Side effects galore
"""
@transition(field=state, source='*', target='destroyed', conditions=[can_destroy])
def destroy(self):
"""
Side effects galore
"""
You can instantiate a field with protected=True option to prevent
direct state field modification.
class BlogPost(models.Model):
state = FSMField(default='new', protected=True)
model = BlogPost()
model.state = 'invalid' # Raises AttributeError
Note that calling refresh_from_db on a model instance with a protected FSMField will cause an exception.
source parameter accepts a list of states, or an individual state or django_state_manager.fsm.State implementation.
You can use * for source to allow switching to target from any state.
You can use + for source to allow switching to target from any state excluding target state.
target state parameter could point to a specific state or django_state_manager.fsm.State implementation
from django_state_manager.fsm import FSMField, transition, RETURN_VALUE, GET_STATE
@transition(field=state,
source='*',
target=RETURN_VALUE('for_moderators', 'published'))
def publish(self, is_public=False):
return 'for_moderators' if is_public else 'published'
@transition(
field=state,
source='for_moderators',
target=GET_STATE(
lambda self, allowed: 'published' if allowed else 'rejected',
states=['published', 'rejected']))
def moderate(self, allowed):
pass
@transition(
field=state,
source='for_moderators',
target=GET_STATE(
lambda self, **kwargs: 'published' if kwargs.get("allowed", True) else 'rejected',
states=['published', 'rejected']))
def moderate(self, allowed=True):
pass
Custom properties can be added by providing a dictionary to the
custom keyword on the transition decorator.
@transition(field=state,
source='*',
target='onhold',
custom=dict(verbose='Hold for legal reasons'))
def legal_hold(self):
"""
Side effects galore
"""
If the transition method raises an exception, you can provide a specific target state
@transition(field=state, source='new', target='published', on_error='failed')
def publish(self):
"""
Some exception could happen here
"""
Instead of passing a two-item iterable choices you can instead use the
three-element state_choices, the last element being a string reference
to a model proxy class.
The base class instance would be dynamically changed to the corresponding Proxy class instance, depending on the state. Even for queryset results, you will get Proxy class instances, even if the QuerySet is executed on the base class.
It is common to have permissions attached to each model transition.
django-state-manager handles this with permission keyword on the
transition decorator. permission accepts a permission string, or
callable that expects instance and user arguments and returns
True if the user can perform the transition.
.. code:: python
@transition(field=state, source='*', target='published',
permission=lambda instance, user: not user.has_perm('myapp.can_make_mistakes'))
def publish(self):
pass
@transition(field=state, source='*', target='removed',
permission='myapp.can_remove_post')
def remove(self):
pass
You can check permission with has_transition_permission method
.. code:: python
from django_state_manager.fsm import has_transition_perm
def publish_view(request, post_id):
post = get_object_or_404(BlogPost, pk=post_id)
if not has_transition_perm(post.publish, request.user):
raise PermissionDenied
post.publish()
post.save()
return redirect('/')
get_all_FIELD_transitions Enumerates all declared transitions
get_available_FIELD_transitions Returns all transitions data
available in current state
get_available_user_FIELD_transitions Enumerates all transitions data
available in current state for provided user
If you store the states in the db table you could use FSMKeyField to ensure Foreign Key database integrity.
In your model :
class DbState(models.Model):
id = models.CharField(primary_key=True, max_length=50)
label = models.CharField(max_length=255)
def __unicode__(self):
return self.label
class BlogPost(models.Model):
state = FSMKeyField(DbState, default='new')
@transition(field=state, source='new', target='published')
def publish(self):
pass
In your fixtures/initial_data.json :
[
{
"pk": "new",
"model": "myapp.dbstate",
"fields": {
"label": "_NEW_"
}
},
{
"pk": "published",
"model": "myapp.dbstate",
"fields": {
"label": "_PUBLISHED_"
}
}
]
Note : source and target parameters in @transition decorator use pk values of DBState model as names, even if field "real" name is used, without _id postfix, as field parameter.
You can also use FSMIntegerField. This is handy when you want to use
enum style constants.
class BlogPostStateEnum(object):
NEW = 10
PUBLISHED = 20
HIDDEN = 30
class BlogPostWithIntegerField(models.Model):
state = FSMIntegerField(default=BlogPostStateEnum.NEW)
@transition(field=state, source=BlogPostStateEnum.NEW, target=BlogPostStateEnum.PUBLISHED)
def publish(self):
pass
django_state_manager.signals.pre_transition and
django_state_manager.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 processed
-
name Transition name
-
source Source model state
-
target Target model state
django-state-manager provides optimistic locking mixin, to avoid concurrent
model state changes. If model state was changed in database
django_state_manager.fsm.ConcurrentTransition exception would be raised on
model.save()
from django_state_manager.fsm import FSMField, ConcurrentTransitionMixin
class BlogPost(ConcurrentTransitionMixin, models.Model):
state = FSMField(default='new')For guaranteed protection against race conditions caused by concurrently executed transitions, make sure:
- Your transitions do not have any side effects except for changes in the database,
- You always run the save() method on the object within
django.db.transaction.atomic()block.
Following these recommendations, you can rely on ConcurrentTransitionMixin to cause a rollback of all the changes that have been executed in an inconsistent (out of sync) state, thus practically negating their effect.
The conditions framework is useful when dealing with authorization or a form of user validation in applications. You can define various conditions as function, and use instances of these classes to manage and combine those conditions flexibly.
Conditions are added to models to check for the availability of certain actions.
The Conditions class inherits from python's built-in list, and it is used to manage a list of functions (which are conditions that need to be checked). It has some key methods:
__add__: This method allows to concatenate new conditions to our current list of conditions. It takes a list as argument and returns a new Conditions object synthesizing the two lists.__get__: This magic method binds the conditions to an instance, making it possible for the conditions to be about that particular instance.__call__: This method attempts to apply all the conditions to the instance. If a ConditionFailed exception occurs, no error is raised at this level. It takes an instance and a user model as parameters.as_bool: This function is similar to call but instead of calling the conditions, it returns a boolean value based on whether the conditions pass or not. If a ConditionFailed exception has been raised it returns False, else it returns True.
The BoundConditions class is responsible for binding the conditions to a certain instance. It has two methods:
__init__: This method initializes the BoundConditions object. It requires two parameters - conditions which is of type Conditions and an instance of any object.__call__: This method forwards a call to the call method of the Conditions class with the instance and user as arguments. It takes a user model as an argument.as_bool: This method simply checks if the conditions linked to the instance, when applied to the user, are respected or not. It also takes a user model as an argument.