How do you separate business rules from the data storage layer in the application?

To separate business logic and data storage layer in a Python application, I use an architectural approach with a clear separation of layers:

• Model — describes the data structure and interaction with the database, for example, through ORM (Django ORM, SQLAlchemy).
• Service layer — implements business rules and data processing logic.
• Controllers or views — handle requests and invoke services.

This separation allows changing business logic without affecting the storage layer and vice versa.

Example using SQLAlchemy and a service layer:

# model.py
from sqlalchemy import Column, Integer, String
from base import Base

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)

# service.py
class UserService:
    def __init__(self, session):
        self.session = session

    def create_user(self, name):
        user = User(name=name)
        self.session.add(user)
        self.session.commit()
        return user

Thus, business rules are concentrated in services, and models only handle structure and data storage.