Where can application business logic be stored…

Business logic can be stored in various places depending on the application's architecture and requirements:

Data Models: In simple applications or when using ORM, part of the logic related to data processing and validation can reside in models.
Services: A common practice is to separate business logic into dedicated service layers or modules. This enhances reusability and simplifies testing.
Controllers/Views: In simple cases, small pieces of business logic can be located here, but this is generally considered an anti-pattern (Fat Controller/View) and makes maintenance harder.
Separate modules/packages: For complex logic or specific business domains, it may be advisable to create separate modules or packages containing only business rules.

When designing, it is recommended to adhere to principles of clean architecture or similar approaches that clearly separate application layers and encapsulate business logic.

An example of a conditional structure with services:

# Project structure
project_root/
├── app/
│   ├── __init__.py
│   ├── models.py      # Data models (ORM)
│   ├── services/
│   │   ├── __init__.py
│   │   ├── user_service.py  # Service for user operations
│   │   └── order_service.py # Service for order operations
│   ├── controllers/
│   │   ├── __init__.py
│   │   ├── user_controller.py # Controller for user requests
│   │   └── order_controller.py # Controller for order requests
│   └── ...
└── config.py           # Configuration

Example of using a service layer:

# services/user_service.py
class UserService:
    def create_user(self, user_data):
        # Logic for creating a user, data validation, database interaction
        pass

    def get_user(self, user_id):
        # Logic for retrieving a user
        pass

# controllers/user_controller.py
from app.services.user_service import UserService

class UserController:
    def __init__(self):
        self.user_service = UserService()

    def handle_create_user_request(self, request_data):
        # Call business logic via service
        self.user_service.create_user(request_data)
        # Handle response

Where can application business logic be stored?