Environment variables

# BAD: Never hardcode sensitive data!
api_key = "sk-1234567890abcdef"  # Everyone can see this!
database = "production_database"  # Can't change without editing code

import os

# Read from environment
api_key = os.environ.get('API_KEY')
database = os.environ.get('DATABASE_NAME', 'default.db')

print(f"Using database: {database}")

# Mac/Linux
export API_KEY=sk-1234567890abcdef
python app.py

# Windows
set API_KEY=sk-1234567890abcdef
python app.py

import os

# Method 1: Get with default
api_key = os.environ.get('API_KEY', 'demo-key')

# Method 2: Check if exists
if 'API_KEY' in os.environ:
    api_key = os.environ['API_KEY']
else:
    print("No API key found")

# Method 3: Will crash if not found
api_key = os.environ['API_KEY']  # KeyError if missing!