Django state field that enforce a workflow path

fields.py

from collections import namedtuple
from functools import wraps
from itertools import chain

from django.utils.functional import curry
from django.db.models import CharField


class StateField(CharField):
    Starts = namedtuple('Starts', ['state'])
    Ends = namedtuple('Ends', ['state'])
    Transition = namedtuple('Transition', ['from_state', 'to_state', 'label'])

    def __init__(self, steps, *args, **kwargs):
        self.steps = steps
        self.transitions = [step for step in self.steps if isinstance(step, StateField.Transition)]
        self.starts_with = [state for state in self.steps if isinstance(state, StateField.Starts)]
        self.ends_with = [state for state in self.steps if isinstance(state, StateField.Ends)]

        kwargs['max_length'] = 255
        kwargs['db_index'] = True
        kwargs['choices'] = self.get_choices()
        super().__init__(*args, **kwargs)

    def get_state_choices(self):
        """Return a choice like list of all available status."""
        choices = list(set(chain(*((step.from_state, step.to_state) for step in self.transitions))))
        return [(choices, choices.title()) for choices in choices]

    @staticmethod
    def before_transition(func):
        """Executed before transition to STATE"""

        def decorator(state):
            @wraps(func)
            def wrapper(self):
                return func(self)

            return wrapper

        return decorator

    @staticmethod
    def after_transition(func):
        """Executed after transition to STATE"""

        def decorator(state):
            @wraps(func)
            def wrapper(self):
                return func(self)

            return wrapper

        return decorator

    def _get_next_states(self, field):
        """Return a choice like list of the available status for the current state."""
        state_position = getattr(self, field.attname)

        return [(transition.to_state, transition.label.title())
                for transition in field.transitions
                if transition.to_state == state_position]

    def contribute_to_class(self, cls, name, virtual_only=False):
        super().contribute_to_class(cls, name, virtual_only)
        setattr(cls, 'get_next_states' % self.name, curry(self._get_next_states, field=self))

    def validate(self, value, model_instance):
        super().validate(value, model_instance)
        # TODO: Validate state transitions

models.py

from django.db import models
from .fields import StateField


class Order(models.Model):
    NEW = 'new'
    CANCELLED = 'cancelled'
    PAYMENT_PENDING = 'payment-pending'
    FULLY_PAID = 'fully-paid'
    SHIPPED = 'shipped'
    RECEIVED = 'received'
    COMPLETED = 'completed'

    user = models.ForeignKey(settings.AUTH_USER_MODEL)

    status = StateField(steps=(
        StateField.Starts(NEW),
        StateField.Transition(NEW, CANCELLED, 'Cancel'),
        StateField.Transition(NEW, PAYMENT_PENDING, 'Confirm Sale'),
        StateField.Transition(PAYMENT_PENDING, CANCELLED, 'Cancel'),
        StateField.Transition(PAYMENT_PENDING, FULLY_PAID, 'Process Payment'),
        StateField.Transition(FULLY_PAID, SHIPPED, 'Seller ships'),
        StateField.Transition(SHIPPED, RECEIVED, 'Buyer received'),
        StateField.Transition(RECEIVED, COMPLETED, 'Completed'),
        StateField.Ends(CANCELLED),
        StateField.Ends(COMPLETED),
    ), default=NEW)