Django friendly finite state machine support
============================================
|Build Status|
django-fsm adds simple declarative state management for django models.
If you need parallel task execution, view and background task code reuse
over different flows - check my new project django-viewflow:
https://github.com/viewflow/viewflow
Instead of adding a state field to a django model and managing its
values by hand, you use ``FSMField`` and mark model methods with
the ``transition`` decorator. These methods could contain side-effects
of the state change.
Nice introduction is available here:
https://gist.github.com/Nagyman/9502133
You may also take a look at django-fsm-admin project containing a mixin
and template tags to integrate django-fsm state transitions into the
django admin.
https://github.com/gadventures/django-fsm-admin
Transition logging support could be achieved with help of django-fsm-log
package
https://github.com/gizmag/django-fsm-log
FSM really helps to structure the code, especially when a new developer
comes to the project. FSM is most effective when you use it for some
sequential steps.
Installation
------------
.. code:: bash
$ pip install django-fsm
Or, for the latest git version
.. code:: bash
$ pip install -e git://github.com/kmmbvnr/django-fsm.git#egg=django-fsm
The library has full Python 3 support
Usage
-----
Add FSMState field to your model
.. code:: python
from django_fsm import FSMField, transition
class BlogPost(models.Model):
state = FSMField(default='new')
Use the ``transition`` decorator to annotate model methods
.. code:: python
@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.
.. code:: python
from django_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
.. code:: python
def can_publish(instance):
# No publishing after 17 hours
if datetime.datetime.now().hour > 17:
return False
return True
Or model methods
.. code:: python
def can_destroy(self):
return self.is_under_investigation()
Use the conditions like this:
.. code:: python
@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.
.. code:: python
class BlogPost(models.Model):
state = FSMField(default='new', protected=True)
model = BlogPost()
model.state = 'invalid' # Raises AttributeError
Note that calling
`refresh_from_db <https://docs.djangoproject.com/en/1.8/ref/models/instances/#django.db.models.Model.refresh_from_db>`_
on a model instance with a protected FSMField will cause an exception.
``source`` state
~~~~~~~~~~~~~~~~
``source`` parameter accepts a list of states, or an individual state or ``django_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 exluding ``target`` state.
``target`` state
~~~~~~~~~~~~~~~~
``target`` state parameter could point to a specific state or ``django_fsm.State`` implementation
.. code:: python
from django_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
~~~~~~~~~~~~~~~~~~~~~
Custom properties can be added by providing a dictionary to the
``custom`` keyword on the ``transition`` decorator.
.. code:: python
@transition(field=state,
source='*',
target='onhold',
custom=dict(verbose='Hold for legal reasons'))
def legal_hold(self):
"""
Side effects galore
"""
``on_error`` state
~~~~~~~~~~~~~~~~~~
If the transition method raises an exception, you can provide a
specific target state
.. code:: python
@transition(field=state, source='new', target='published', on_error='failed')
def publish(self):
"""
Some exception could happen here
"""
``state_choices``
~~~~~~~~~~~~~~~~~
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.
Check the `test
case <https://github.com/kmmbvnr/django-fsm/blob/master/tests/testapp/tests/test_state_transitions.py>`__
for example usage. Or read about `implementation
internals <http://schinckel.net/2013/06/13/django-proxy-model-state-machine/>`__
Permissions
~~~~~~~~~~~
It is common to have permissions attached to each model transition.
``django-fsm`` 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_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('/')
Model methods
~~~~~~~~~~~~~
``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
Foreign Key constraints support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you store the states in the db table you could use FSMKeyField to
ensure Foreign Key database integrity.
In your model :
.. code:: python
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 :
.. code:: 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.
Integer Field support
~~~~~~~~~~~~~~~~~~~~~
You can also use ``FSMIntegerField``. This is handy when you want to use
enum style constants.
.. code:: python
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
Signals
~~~~~~~
``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 processed
**name** Transition name
**source** Source model state
**target** Target model state
Optimistic locking
------------------
``django-fsm`` provides optimistic locking mixin, to avoid concurrent
model state changes. If model state was changed in database
``django_fsm.ConcurrentTransition`` exception would be raised on
model.save()
.. code:: python
from django_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.
Drawing transitions
-------------------
Renders a graphical overview of your models states transitions
You need ``pip install graphviz>=0.4`` library and add ``django_fsm`` to
your ``INSTALLED_APPS``:
.. code:: python
INSTALLED_APPS = (
...
'django_fsm',
...
)
.. code:: bash
# 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
Changelog
---------
django-fsm 2.8.1 2022-08-15
~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Improve fix for get_available_FIELD_transition
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