Django friendly finite state machine support

Django FSM-2 adds simple, declarative state management to Django models.

Introduction

FSM really helps to structure the code, and centralize the lifecycle of your Models.

Instead of adding a CharField field to a django model and manage its values by hand everywhere, FSMFields offer the ability to declare your transitions once with the decorator. These methods could contain side-effects, permissions, or logic to make the lifecycle management easier.

Nice introduction is available here: https://gist.github.com/Nagyman/9502133

Quick start

from django.db import models
import django_fsm as fsm


class BlogPost(fsm.FSMModelMixin, models.Model):
    state = fsm.FSMField(default="new")

    @fsm.transition(field=state, source="new", target="published")
    def publish(self, **kwargs):
        pass

import django_fsm as fsm

post = BlogPost.objects.get(pk=1)
if fsm.can_proceed(post.publish):
    post.publish()
    post.save()

Installation

Install the package:

uv pip install django-fsm-2

Or install from git:

uv pip install -e git://github.com/django-commons/django-fsm-2.git#egg=django-fsm

Add django_fsm to your Django apps (required to graph transitions or use Admin integration):

INSTALLED_APPS = (
    ...,
    "django_fsm",
    ...,
)

Django FSM-2 is a drop-in replacement. Update your dependency from django-fsm to django-fsm-2 and keep your existing code.

uv pip install django-fsm-2

Usage

Core ideas

• Store a state in an FSMField (or FSMIntegerField/FSMKeyField).
• Declare transitions once with the @fsm.transition decorator.
• Transition methods can contain business logic and side effects.
• The in-memory state changes on success; save() persists it.

Adding an FSM field

import django_fsm as fsm


class BlogPost(fsm.FSMModelMixin, models.Model):
    state = fsm.FSMField(default="new")

Declaring a transition

import django_fsm as fsm


@fsm.transition(field=state, source="new", target="published")
def publish(self, **kwargs):
    """
    This function may contain side effects,
    like updating caches, notifying users, etc.
    The return value will be discarded.
    """

The field parameter accepts a string attribute name or a field instance. If calling publish() succeeds without raising an exception, the state changes in memory. You must call save() to persist it.

import django_fsm as fsm


def publish_view(request, post_id, **kwargs):
    post = get_object_or_404(BlogPost, pk=post_id)
    if not fsm.can_proceed(post.publish):
        raise PermissionDenied

    post.publish()
    post.save()
    return redirect("/")

Preconditions (conditions)

Use conditions to restrict transitions. Each function receives the instance and must return truthy/falsey. The functions should not have side effects.

import django_fsm as fsm


def can_publish(instance):
    # No publishing after 17 hours
    return datetime.datetime.now().hour <= 17


class XXX(fsm.FSMModelMixin, models.Model):
    @fsm.transition(field=state, source="new", target="published", conditions=[can_publish])
    def publish(self, **kwargs):
        pass

You can also use model methods:

import django_fsm as fsm


class XXX(fsm.FSMModelMixin, models.Model):
    def can_destroy(self):
        return self.is_under_investigation()

    @fsm.transition(field=state, source="*", target="destroyed", conditions=[can_destroy])
    def destroy(self, **kwargs):
        pass

When a transition is blocked by an unmet condition, the raised TransitionConditionsUnmet exception includes the failing callable in its failed_condition attribute for programmatic handling:

from django_fsm import TransitionConditionsUnmet

try:
    instance.publish()
except TransitionConditionsUnmet as e:
    print(e.failed_condition)  # e.g. <function can_publish at 0x...>

Protected state fields

Use protected=True to prevent direct assignment. Only transitions may change the state.

Because refresh_from_db assigns to the field, protected fields raise there as well unless you use FSMModelMixin. Use FSMModelMixin by default to allow refresh without enabling arbitrary writes elsewhere.

import django_fsm as fsm


class BlogPost(fsm.FSMModelMixin, models.Model):
    state = fsm.FSMField(default="new", protected=True)


model = BlogPost()
model.state = "invalid"  # Raises AttributeError
model.refresh_from_db()  # Works

Source and target states

source accepts a list of states, a single state, or a django_fsm.State implementation.

• source="*" allows switching to target from any state (accessible as ANY_STATE constant).
• source="+" allows switching to target from any state except target(accessible as ANY_OTHER_STATE constant).

target can be a specific state or a django_fsm.State implementation.

import django_fsm as fsm


@fsm.transition(
    field=state,
    source="*",
    target=fsm.RETURN_VALUE("for_moderators", "published"),
)
def publish(self, is_public=False, **kwargs):
    return "for_moderators" if is_public else "published"


@fsm.transition(
    field=state,
    source="for_moderators",
    target=fsm.GET_STATE(
        lambda self, allowed: "published" if allowed else "rejected",
        states=["published", "rejected"],
    ),
)
def moderate(self, allowed, **kwargs):
    pass


@fsm.transition(
    field=state,
    source="for_moderators",
    target=fsm.GET_STATE(
        lambda self, **kwargs: "published" if kwargs.get("allowed", True) else "rejected",
        states=["published", "rejected"],
    ),
)
def moderate(self, allowed=True, **kwargs):
    pass

Custom transition metadata

Use custom to attach arbitrary data to a transition.

@fsm.transition(
    field=state,
    source="*",
    target="onhold",
    custom=dict(verbose="Hold for legal reasons"),
)
def legal_hold(self, **kwargs):
    pass

Error target state

If a transition method raises an exception, you can specify an on_error state.

@fsm.transition(field=state, source="new", target="published", on_error="failed")
def publish(self, **kwargs):
    """
    Some exception could happen here
    """

Permissions

Attach permissions to transitions with the permission argument. It accepts a permission string or a callable that receives (instance, user).

@fsm.transition(
    field=state,
    source="*",
    target="published",
    permission=lambda instance, user: not user.has_perm("myapp.can_make_mistakes"),
)
def publish(self, **kwargs):
    pass


@fsm.transition(
    field=state,
    source="*",
    target="removed",
    permission="myapp.can_remove_post",
)
def remove(self, **kwargs):
    pass

Check permission with has_transition_perm:

import django_fsm as fsm


def publish_view(request, post_id):
    post = get_object_or_404(BlogPost, pk=post_id)
    if not fsm.has_transition_perm(post.publish, request.user):
        raise PermissionDenied

    post.publish()
    post.save()
    return redirect("/")

Model helpers

Considering a model with a state field called "FIELD"

• get_all_FIELD_transitions enumerates all declared transitions.
• get_available_FIELD_transitions returns transitions available in the current state.
• get_available_user_FIELD_transitions returns transitions available in the current state for a given user.

Example: If your state field is called status

my_model_instance.get_all_status_transitions()
my_model_instance.get_available_status_transitions()
my_model_instance.get_available_user_status_transitions()

FSMKeyField (foreign key support)

Use FSMKeyField to store state values in a table and maintain FK integrity.

import django_fsm as fsm


class DbState(fsm.FSMModelMixin, models.Model):
    id = models.CharField(primary_key=True)
    label = models.CharField()

    def __str__(self):
        return self.label


class BlogPost(fsm.FSMModelMixin, models.Model):
    state = fsm.FSMKeyField(DbState, default="new")

    @fsm.transition(field=state, source="new", target="published")
    def publish(self, **kwargs):
        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 use the PK values of the DbState model as names, even if the field is accessed without the _id postfix.

FSMIntegerField (enum-style states)

import django_fsm as fsm


class BlogPostStateChoices(models.IntegerChoices):
    NEW = 10, "New"
    PUBLISHED = 20, "Published"
    HIDDEN = 30, "Hidden"


class BlogPostWithIntegerField(fsm.FSMModelMixin, models.Model):
    state = fsm.FSMIntegerField(default=BlogPostStateChoices.NEW)

    @fsm.transition(
        field=state,
        source=BlogPostStateChoices.NEW,
        target=BlogPostStateChoices.PUBLISHED,
    )
    def publish(self, **kwargs):
        pass

Signals

django_fsm.signals.pre_transition and django_fsm.signals.post_transition fire before and after an allowed transition. No signals fire for invalid transitions.

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

Use ConcurrentTransitionMixin to avoid concurrent state changes. If the state changed in the database, django_fsm.ConcurrentTransition is raised on save().

import django_fsm as fsm


class BlogPost(fsm.ConcurrentTransitionMixin, models.Model):
    state = fsm.FSMField(default="new")

For guaranteed protection against race conditions caused by concurrently executed transitions, make sure:

• Your transitions do not have side effects except for database changes.
• You always call save() within a django.db.transaction.atomic() block.

Following these recommendations, ConcurrentTransitionMixin will cause a rollback of all changes executed in an inconsistent state.

Admin Integration

NB: If you're migrating from django-fsm-admin (or any alternative), make sure it's not installed anymore to avoid installing the old django-fsm.

Update import path:

- from django_fsm_admin.mixins import FSMTransitionMixin
+ from django_fsm.admin import FSMAdminMixin

1. In your admin.py file, use FSMAdminMixin to add behaviour to your ModelAdmin. FSMAdminMixin should be before ModelAdmin, the order is important.

from django_fsm.admin import FSMAdminMixin


@admin.register(AdminBlogPost)
class MyAdmin(FSMAdminMixin, admin.ModelAdmin):
    # Declare the fsm fields you want to manage
    fsm_fields = ["my_fsm_field"]
    ...

2. You can customize the buttons by adding label and help_text to the custom attribute of the transition decorator

@fsm.transition(
    field="state",
    source=["startstate"],
    target="finalstate",
    custom={
        "label": "My awesome transition",  # this
        "help_text": "Rename blog post",  # and this
    },
)
def do_something(self, **kwargs): ...

or by overriding some methods in FSMAdminMixin

from django_fsm.admin import FSMAdminMixin


@admin.register(AdminBlogPost)
class MyAdmin(FSMAdminMixin, admin.ModelAdmin):
    ...

    def get_fsm_label(self, transition):  # this method
        if transition.name == "do_something":
            return "My awesome transition"
        return super().get_fsm_label(transition)

    def get_help_text(self, transition):  # and this method
        if transition.name == "do_something":
            return "Rename blog post"
        return super().get_help_text(transition)

3. For forms in the admin transition flow, see the Custom Forms section below.

4. Hiding a transition is possible by adding custom={"admin": False} to the transition decorator:

    @fsm.transition(
        field='state',
        source=['startstate'],
        target='finalstate',
        custom={
            "admin": False,  # this
        },
    )
    def do_something(self, **kwargs):
       # will not add a button "Do Something" to your admin model interface

or from the admin:

from django_fsm.admin import FSMAdminMixin


@admin.register(AdminBlogPost)
class MyAdmin(FSMAdminMixin, admin.ModelAdmin):
    ...

    def is_fsm_transition_visible(self, transition: fsm.Transition) -> bool:
        if transition.name == "do_something":
            return False
        return super().is_fsm_transition_visible(transition)

NB: By adding FSM_ADMIN_FORCE_PERMIT = True to your configuration settings (or fsm_default_disallow_transition = False to your admin), the above restriction becomes the default. Then one must explicitly allow that a transition method shows up in the admin interface using custom={"admin": True}

from django_fsm.admin import FSMAdminMixin


@admin.register(AdminBlogPost)
class MyAdmin(FSMAdminMixin, admin.ModelAdmin):
    fsm_default_disallow_transition = False
    ...

Custom Forms

You can attach a custom form to a transition so the admin prompts for input before the transition runs. Add a form entry to custom on the transition, or define an admin-level mapping via fsm_forms. Both accept a forms.Form/ forms.ModelForm class or a dotted import path.

from django import forms
import django_fsm as fsm


class RenameForm(forms.Form):
    new_title = forms.CharField(max_length=255)
    # it's also possible to declare fsm log description
    description = forms.CharField(max_length=255)


class BlogPost(fsm.FSMModelMixin, models.Model):
    title = models.CharField(max_length=255)
    state = fsm.FSMField(default="created")

    @fsm.transition(
        field=state,
        source="*",
        target="created",
        custom={
            "label": "Rename",
            "help_text": "Rename blog post",
            "form": "path.to.RenameForm",
        },
    )
    def rename(self, new_title, **kwargs):
        self.title = new_title

You can also define forms directly on your ModelAdmin without touching the transition definition:

from django_fsm.admin import FSMAdminMixin

from .admin_forms import RenameForm


@admin.register(AdminBlogPost)
class MyAdmin(FSMAdminMixin, admin.ModelAdmin):
    fsm_fields = ["state"]
    fsm_forms = {
        "rename": "path.to.RenameForm",  # use import path
        "rename": RenameForm,  # or FormClass
    }

Behavior details:

• When form is set, the transition button redirects to a form view instead of executing immediately.
• If both are defined, fsm_forms on the admin takes precedence over custom["form"] on the transition.
• On submit, cleaned_data is passed to the transition method as keyword arguments and the object is saved.
• RenameForm receives the current instance automatically.
• You can override the transition form template by setting fsm_transition_form_template on your ModelAdmin (or override globally templates/django_fsm/fsm_admin_transition_form.html).

Unfold support

If you use Django Unfold, this package provide a contrib that contains templates tailored to the Unfold admin UI.

INSTALLED_APPS = (
    ...,
    "unfold",
    "django_fsm.contrib.unfold",
    ...,
)

Drawing transitions

Render a graphical overview of your model transitions.

1. Install graphviz support:

uv pip install django-fsm-2[graphviz]

or

uv pip install "graphviz>=0.4"

2. Ensure django_fsm is in INSTALLED_APPS:

INSTALLED_APPS = (
    ...,
    "django_fsm",
    ...,
)

3. Run the management command:

# Create a dot file
./manage.py graph_transitions > transitions.dot

# Create a PNG image file for a specific model
./manage.py graph_transitions -o blog_transitions.png myapp.Blog

# Exclude some transitions
./manage.py graph_transitions -e transition_1,transition_2 myapp.Blog

Styling the graph

graph_transitions's Command is designed to be subclassed, so you can color or style the exported diagram by overriding a couple of hook methods. Subclass the command from an app listed before django_fsm in INSTALLED_APPS:

# myapp/management/commands/graph_transitions.py
from django_fsm.management.commands.graph_transitions import Command as BaseCommand

STATE_COLORS = {"approved": "green", "rejected": "red"}


class Command(BaseCommand):
    def get_transition_edge_attrs(self, transition):
        if transition.name == "approve":
            return {"color": "green", "penwidth": "2"}
        if color := transition.custom.get("graph_color"):
            return {"color": color, "penwidth": "2"}
        return super().get_transition_edge_attrs(transition)

    def get_state_node_attrs(self, field, state, label):
        if color := STATE_COLORS.get(state):
            return {"style": "filled", "fillcolor": color}
        return super().get_state_node_attrs(field, state, label)

Extensions

Transition logging support could be achieved with help of django-fsm-log package : https://github.com/gizmag/django-fsm-log

Contributing

We welcome contributions. See CONTRIBUTING.md for detailed setup instructions.

Quick Development Setup

# Clone and setup
git clone https://github.com/django-commons/django-fsm-2.git
cd django-fsm
uv sync

# Run tests
uv run pytest -v
# or
uv run tox

# Run linting
uv run ruff format .
uv run ruff check .