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Django Logic is a workflow framework allowing developers to implement the business logic via pure functions. It's designed based on Finite-State-Machine (FSM) principles. Therefore, it needs to define a state field for a model's object. Every change of the state is performed by a transition and every transition could be grouped into a process. Also, you can define some side-effects that will be executed during the transition from one state to another and callbacks that will be run after. This concept provides you a place for the business logic, rather than splitting it across the views, models, forms, serializers or even worse, in templates.

Definitions

• Transition - class changes a state of an object from one to another. It also contains its own conditions, permissions, side-effects, callbacks, and failure callbacks.
• Action - in contrast with the transition, the action does not change the state. But it contains its own conditions, permissions, side-effects, callbacks, and failure callbacks.
• Side-effects - class defines a set of functions that executing within one particular transition before reaching the target state. During the execution, the state changes to the in_progress state. In case, if one of the functions interrupts the execution, then it changes to the failed state.
• Callbacks - class defines a set of functions that executing within one particular transition after reaching the target state. In case, if one of the functions interrupts the execution, it will log an exception and the execution will be stopped (without changing the state to failed).
• Failure callbacks - class defines a set of functions that executing within one particular transition in case if one of the side-effects has been failed to execute.
• Conditions - class defines a set of functions which receives an object and return True or False based on one particular requirement.
• Permissions - class defines a set of functions which receives an object and user, then returns True or False based on given permissions.
• Process - class defines a set of transitions with some common conditions and permissions. It also accepts nested processes that allow building the hierarchy.

Installation

Use the package manager pip to install Django-Logic.

pip install django-logic

Usage

0. Add to INSTALLED_APPS

INSTALLED_APPS = (
    ...
    'django_logic',
    ...
)

1. Define a process class with some transitions.

from django_logic import Process as BaseProcess, Transition, ProcessManager, Action


class Process(BaseProcess):
    states = (
        ('draft', 'Draft'),
        ('approved', 'Approved'),
        ('void', 'Void'),
    )
    transitions = [
        Transition(action_name='approve', sources=['draft'], target='approved'),
        Transition(action_name='void', sources=['draft', 'approved'], target='void'),
        Action(action_name='update', side_effects=[update_data]),
    ]

2. Define a binding class and status field.

ApprovalProcess = ProcessManager.bind_state_fields(status=Process)

You can call the status or state field as you wish, just make sure it's defined in the model. Furthermore, it supports several state fields. For example:

ApprovalProcess = ProcessManager.bind_state_fields(my_status=ApprovalProcess, my_state=LockProcess)

class Invoice(ApprovalProcess, models.Model):
    my_status = models.CharField(choices=ApprovalProcess.states, default='draft', max_length=16, blank=True)
    my_state = models.CharField(choices=LockProcess.states, default='open', max_length=16, blank=True)

3. Bind the process with a model by inheriting the binding class.

from django.db import models
from .process import ApprovalProcess

class Invoice(ApprovalProcess, models.Model):
    status = models.CharField(choices=ApprovalProcess.states, default='draft', max_length=16, blank=True)

4. Advance your process with conditions, side-effects, and callbacks into the process

class Process(BaseProcess):
    permissions = [
        is_accountant, 
    ]
    states = (
        ('draft', 'Draft'),
        ('approved', 'Approved'),
        ('void', 'Void'),
    )
    transitions = [
        Transition(
            action_name='approve',
            sources=['draft'], 
            target='approved',
            conditions=[
                is_customer_active, 
            ]
            side_effects=[
                generate_pdf_invoice, 
            ],
            callbacks=[
                send_approved_invoice_email_to_accountant, 
            ]
        ),
        Transition(
            action_name='void', 
            callbacks=[
                send_void_invoice_email_to_accountant
            ],
            sources=['approved'],
            target='void'
        ),
        Action(
            action_name='update', 
            side_effects=[
                update_data
            ],
        ),
    ]

5. This approval process defines the business logic where:

• The user who performs the action must have accountant role (permission).
• It shouldn't be possible to invoice inactive customers (condition).
• Once the invoice record is approved, it should generate a PDF file and send it to an accountant via email. (side-effects and callbacks)
• If the invoice voided it needs to notify the accountant about that. As you see, these business requirements should not know about each other. Furthermore, it gives a simple way to test every function separately as Django-Logic takes care of connection them into the business process.

6. Execute in the code:

from invoices.models import Invoice


def approve_view(request, pk):
    invoice = Invoice.objects.get(pk=pk)
    invoice.process.approve(user=request.user)

7. If you want to override the value of the state field, it must be done explicitly. For example:

Invoice.objects.filter(status='draft').update(status='open')
# or 
invoice = Invoice.objects.get(pk=pk)
invoice.status = 'open'
invoice.save(update_fields=['status'])

Save without update_fields won't update the value of the state field in order to protect the data from corrupting.

8. Error handling:

from django_logic.exceptions import TransitionNotAllowed

try:
    invoice.process.approve()
except TransitionNotAllowed:
    logger.error('Approve is not allowed') 

Display process

Drawing a process with the following elements:

• Process - a transparent rectangle
• Transition - a grey rectangle
• State - a transparent ellipse
• Process' conditions and permissions are defined inside of related process as a transparent diamond
• Transition' conditions and permissions are defined inside of related transition's process as a grey diamond

From this diagram you can visually check that the following the business requirements have been implemented properly:

• Personnel involved: User and Staff
• Lock has to be available before any actions taken. It's defined by a condition is_lock_available.
• User is able to lock and unlock an available locker.
• Staff is able to lock, unlock and put a locker under maintenance if such was planned.

Drawing such diagram requires installing graphviz.

pip install graphviz

Run this command

from django_logic.display import * 
from demo.process import LockerProcess
display_process(LockerProcess, state='open', skip_main_process=True)

Django-Logic vs Django FSM

Django FSM is a parent package of Django-Logic. It's been used in production for many years until the number of new ideas and requirements swamped us. Therefore, it's been decided to implement these ideas under a new package. For example, supporting Processes or background transitions which were implemented under Django-Logic-Celery. Finally, we want to provide a standard way on where to put the business logic in Django by using Django-Logic.

Contributing

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

Please make sure to update tests as appropriate.

License

MIT

Project status

Under development