Python Environment Variables

Environment variables are settings stored outside your code.

        Examples:
        - DB_HOST
        - API_KEY
        - SECRET_KEY
        - DEBUG mode
        
        ### Why they matter
        If you hardcode secrets in code:
        - they can leak in GitHub
        - they are hard to change per environment
        - your app becomes unsafe
        
        Environment variables solve that.
        
        ## Read environment variables
        
        ```python
        import os
        
        db_host = os.environ.get("DB_HOST", "localhost")
        db_port = os.environ.get("DB_PORT", "5432")
        
        print(f"Connecting to {db_host}:{db_port}")
        ```
        
        ## Setting environment variables (concept)
        
        You typically set env vars in:
        - terminal
        - Docker
        - CI/CD pipeline
        - server environment
        
        Example (Linux/macOS):
        ```bash
        export DB_HOST=localhost
        ```
        
        Example (Windows PowerShell):
        ```powershell
        $env:DB_HOST="localhost"
        ```
        
        ## Using .env file (common in development)
        
        ```bash
        pip install python-dotenv
        ```
        
        Create `.env`:
        ```
        DB_HOST=localhost
        DB_PORT=5432
        API_KEY=secret123
        ```
        
        Load it:
        ```python
        from dotenv import load_dotenv
        import os
        
        load_dotenv()
        
        db_host = os.getenv("DB_HOST")
        api_key = os.getenv("API_KEY")
        print(db_host)
        ```
        
        ## Config class pattern (clean and organized)
        
        ```python
        import os
        from dotenv import load_dotenv
        
        load_dotenv()
        
        class Config:
            DB_HOST = os.getenv("DB_HOST", "localhost")
            DB_PORT = int(os.getenv("DB_PORT", "5432"))
            API_KEY = os.getenv("API_KEY")
            DEBUG = os.getenv("DEBUG", "False").lower() == "true"
        
        config = Config()
        print(config.DB_HOST, config.DB_PORT)
        ```
        
        ## Graph: config flow
        
        ```mermaid
        flowchart LR
          A[.env / server env vars] --> B[os.getenv]
          B --> C[Config class]
          C --> D[App uses settings safely]
        ```
        
        ## Remember
        
        - Never commit .env files to Git
        - Keep secrets in env vars
        - Use defaults for non-secret config
        - Different env vars for dev vs production