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django-fsm 2.2.0

Django friendly finite state machine support.

Django friendly finite state machine support
============================================
[![Build Status]https://travis-ci.org/kmmbvnr/django-fsm.svg?branch=master]https://travis-ci.org/kmmbvnr/django-fsm

django-fsm adds declarative states management for django models.

Instead of adding some state field to a django model, and managing its
values by hand, you could use FSMState field and mark model methods
with the `transition` decorator. Your method could contain the 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 achived 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.

If you need parallel task execution, view and background task code reuse
over different flows - check my new project django-viewflow

https://github.com/kmmbvnr/django-viewflow

Installation
------------
```bash
$ pip install django-fsm
```
Or, for the latest git version
```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
```python
from django_fsm import FSMField, transition

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

Use the `transition` decorator to annotate model methods
```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.
"""
```

`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.
```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
```python
def can_publish(instance):
# No publishing after 17 hours
if datetime.datetime.now().hour > 17:
return False
return True
```

Or model methods
```python
def can_destroy(self):
return self.is_under_investigation()
```

Use the conditions like this:
```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 could instantiate a field with protected=True option, that prevents direct state field modification
```python
class BlogPost(models.Model):
state = FSMField(default='new', protected=True)

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


### `custom` properties
Custom properties can be added by providing a dictionary to the `custom` keyword on the `transition` decorator.
```python
@transition(field=state,
source='*',
target='onhold',
custom=dict(verbose='Hold for legal reasons'), transition_type='manual')
def legal_hold(self):
"""
Side effects galore
"""
```

### `on_error` state

In case of transition method would raise exception, you can provide specific target state

```python
@transition(field=state, source='new', target='published', on_error='failed')
def publish(self):
"""
Some exceptio could happends here
"""
```

### `state_choices`
Instead of passing two elements list `choices` you could use three elements `state_choices`,
the last element states for string reference to model proxy class.

Base class instance would be dynamically changed to corresponding Proxy class instance, depending on the state.
Even for queryset results, you will get Proxy class instances, even if QuerySet executed on 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 `user` argument and returns True if user can perform the transition

```python
@transition(field=state, source='*', target='publish',
permission=lambda user: not user.has_perm('myapp.can_make_mistakes'))
def publish(self):
pass

@transition(field=state, source='*', target='publish',
permission='myapp.can_remove_post')
def remove(self):
pass
```

You can check permission with `has_transition_permission` method

``` 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 :
```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 :
```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.
```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 procceed

**name**
Transition name

**source**
Source model state

**target**
Target model state

## Optimistic locking

`django-fsm` provides optimistic locking mixin, to avoid concurent model state changes.
If model state was changed in database `django_fsm.ConcurrentTransition` exception would be raised
on model.save()

```python
from django_fsm import FMSField, 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

```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 2.2.0 2014-09-03
* Support for [class substitution]http://schinckel.net/2013/06/13/django-proxy-model-state-machine/ to proxy classes depending on the state
* Added ConcurrentTransitionMixin with optimistic locking support
* Default db_index=True for FSMIntegerField removed
* Graph transition code migrated to new graphviz library with python 3 support
* Ability to change state on transition exception  
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