How to Mock Environment Variables in Python for 12-Factor Config

import os
import json
from unittest.mock import patch

def load_config(env_prefix="APP"):
    """Read 12-factor style config from env vars"""
    required = ["DATABASE_URL", "API_KEY"]
    optional = {"PORT": "8080", "DEBUG": "false"}
    
    config = {}
    for key in required:
        full_key = f"{env_prefix}_{key}"
        value = os.environ.get(full_key)
        if value is None:
            raise ValueError(f"Missing required env var: {full_key}")
        config[key] = value
    
    for key, default in optional.items():
        full_key = f"{env_prefix}_{key}"
        config[key] = os.environ.get(full_key, default)
    
    return config

if __name__ == "__main__":
    # Mock environment vars without touching real environment
    mock_env = {
        "APP_DATABASE_URL": "postgres://localhost/mydb",
        "APP_API_KEY": "secret-key-123",
        "APP_DEBUG": "true",
        "APP_PORT": "3000"
    }
    
    with patch.dict(os.environ, mock_env):
        config = load_config()
        print(json.dumps(config, indent=2))
    
    # Without mock, should fail on missing required vars
    try:
        config = load_config()
        print(config)
    except ValueError as e:
        print(f"Error: {e}")