Python Logging

Logging is how real applications “talk” about what they are doing.

        Why logging is important:
        - in production you cannot rely on print()
        - logs help you debug issues after they happen
        - logs help you see user behavior and system failures
        
        ## Logging levels (simple meaning)
        
        - DEBUG: detailed developer info
        - INFO: normal operations
        - WARNING: something unexpected but not fatal
        - ERROR: operation failed
        - CRITICAL: app is in danger
        
        ## Basic logging
        
        ```python
        import logging
        
        logging.basicConfig(level=logging.INFO)
        
        logging.debug("Debug message")
        logging.info("Info message")
        logging.warning("Warning message")
        logging.error("Error message")
        logging.critical("Critical message")
        ```
        
        ## Log to a file (production friendly)
        
        ```python
        import logging
        
        logging.basicConfig(
            filename="app.log",
            level=logging.INFO,
            format="%(asctime)s - %(levelname)s - %(message)s"
        )
        
        logging.info("Application started")
        logging.error("An error occurred")
        ```
        
        ## Multiple handlers (console + file)
        
        ```python
        import logging
        
        logger = logging.getLogger(__name__)
        logger.setLevel(logging.DEBUG)
        
        console_handler = logging.StreamHandler()
        console_handler.setLevel(logging.INFO)
        
        file_handler = logging.FileHandler("errors.log")
        file_handler.setLevel(logging.ERROR)
        
        formatter = logging.Formatter(
            "%(asctime)s - %(name)s - %(levelname)s - %(message)s"
        )
        console_handler.setFormatter(formatter)
        file_handler.setFormatter(formatter)
        
        logger.addHandler(console_handler)
        logger.addHandler(file_handler)
        
        logger.info("Visible in console")
        logger.error("Saved in errors.log and also shown if console level allows")
        ```
        
        ## Log full exception details
        
        ```python
        import logging
        
        logging.basicConfig(level=logging.ERROR)
        
        try:
            10 / 0
        except Exception:
            logging.error("Crash happened", exc_info=True)
        ```
        
        ## Graph: where logs go
        
        ```mermaid
        flowchart LR
          A[Your code] --> B[logger.info/error]
          B --> C[Console]
          B --> D[Log file]
          D --> E[Monitoring / Debugging]
        ```
        
        ## Remember
        
        - Use logging instead of print() in real apps
        - Log files are critical for production debugging
        - Never log secrets (passwords, tokens, API keys)