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Django friendly finite state machine support

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Django friendly finite state machine support

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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 excluding ``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

.. |Build Status| image:: https://travis-ci.org/viewflow/django-fsm.svg?branch=master :target: https://travis-ci.org/viewflow/django-fsm

Open Source Agenda is not affiliated with "Django Fsm" Project. README Source: viewflow/django-fsm
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