The Singleton pattern restricts a class to one instance and provides a global access point to that shared object. It suits logging, caching, configuration, and database connection management. section 4.3.3
A practical use is replacing an in-memory repository with database persistence while keeping one shared connection manager. Database settings belong in environment variables rather than source code, and SQLAlchemy supplies the database integration. section 4.3.3 section 4.3.4
Database connection manager
A DatabaseConnection class stores a private instance, exposes get_instance(), and rejects construction after the first instance exists. Its constructor creates a SQLAlchemy engine and session factory from DB_USERNAME, DB_PASSWORD, DB_HOST, DB_PORT, and DB_NAME. section 4.3.4
Review the implementation before applying it:
- Keep usernames, passwords, and connection details out of source code; configure them per environment. section 4.3.4
- Wire the connection manager, repository, service, adapter, and controller in the application entry point so components remain replaceable. section 4.3.4
Listing 4.29 Implementation of a DatabaseConnection
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
import os
class DatabaseConnection:
__instance = None
@staticmethod
def get_instance():
if DatabaseConnection.__instance == None:
DatabaseConnection()
return DatabaseConnection.__instance
def __init__(self):
if DatabaseConnection.__instance != None:
raise Exception("This class is a singleton!")
else:
DatabaseConnection.__instance = self
self.engine =
create_engine(f"postgresql://
{os.environ['DB_USERNAME']}:
{os.environ['DB_PASSWORD']}@
{os.environ['DB_HOST']}:
{os.environ['DB_PORT']}/
{os.environ['DB_NAME']}")
self.Session = sessionmaker(bind=self.engine)
def get_session(self):
return self.Session()
Inspect the complete Python implementation of the singleton database connection, including instance access, construction protection, engine creation, and session generation. section 4.3.4
