config.py

import os

class Config:
    """Base configuration."""
    SECRET_KEY = os.environ.get('SECRET_KEY') or 'you-will-never-guess'
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URI') or 'sqlite:///app.db'
    SQLALCHEMY_TRACK_MODIFICATIONS = False

class DevelopmentConfig(Config):
    """Development configuration."""
    DEBUG = True

class TestingConfig(Config):
    """Testing configuration."""
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite:///:memory:' # Use in-memory SQLite database for tests

class ProductionConfig(Config):
    """Production configuration."""
    # In production, SECRET_KEY and DATABASE_URI must be set as environment variables.
    # The application will fail to start if they are not set.
    SECRET_KEY = os.environ.get('SECRET_KEY')
    DATABASE_URI = os.environ.get('DATABASE_URI')

config = {
    'development': DevelopmentConfig,
    'testing': TestingConfig,
    'production': ProductionConfig,
    'default': DevelopmentConfig
}