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> In functions decorated with errorhandler, the return must explicitly include the status code. If you don't write , 404, Flask will return status code 200 by default, and browsers and search engines will not recognize this as an error page. * * * ## Exception Class Registration In addition to status codes, @app.errorhandler can also register exception classes directly: ## Example from werkzeug.exceptions import HTTPException @app.errorhandler(HTTPException) def handle_http_exception(error): """Handle all HTTP exceptions uniformly (such as 400, 401, 403, 404, etc.)""" return f"""HTTP Error {error.code}
{error.description}
""", error.code * * * ## Using abort to Trigger Errors In your view functions, use abort() to actively trigger HTTP errors: ## Example from flask import abort # Simulate an article database articles ={ 1: {"title": "Flask Getting Started Tutorial"}, 2: {"title": "Python Basics"}, } @app.get("/article/") def view_article(article_id): article = articles.get(article_id) if article is None: # Article does not exist, return 404 # description parameter will be passed to errorhandler abort(404, description=f"Article ID {article_id} does not exist") return f"{article['title']}
" @app.get("/admin") def admin_panel(): # Users without admin permission access the admin backend abort(403, description="You do not have administrator permissions") * * * ## Logging Flask uses Python's standard library logging module, and you can record logs through app.logger. Logs are essential for troubleshooting - don't just use print(), logs provide richer context and better control. ## Example from flask import Flask, request app = Flask( __name__ ) @app.route("/") def index(): # Different levels of logs app.logger.debug("Accessing homepage")# Debug info, usually not output in production app.logger.info("User from IP: %s", request.remote_addr)# General info app.logger.warning("Abnormal access pattern detected")# Warning: needs attention but doesn't affect operation app.logger.error("Database connection failed")# Error: functionality affected app.logger.critical("Insufficient disk space, service will stop soon")# Critical: service may stop return"OK" Log levels (from low to high): | Level | Use Case | | --- | --- | | DEBUG | Detailed information during development and debugging, not output by default in production | | INFO | General runtime information, such as request records, service startup | | WARNING | Warning information, potential issues but doesn't affect current operation | | ERROR | Error information, some functionality failed but service is still running | | CRITICAL | Critical error, may cause service to stop | > In Debug mode, Flask automatically sets the log level to DEBUG. In production, it should be set to INFO or WARNING as needed to avoid outputting too much useless information. * * * ## Configure Log Output Write logs to files for later analysis and troubleshooting: ## Example import logging from logging.handlers import RotatingFileHandler # Configure file log handler - log file automatically rotates after reaching 10MB handler = RotatingFileHandler( "tutorial_app.log",# Log file path maxBytes=10 * 1024 * 1024,# Maximum 10MB per file backupCount=5# Keep the last 5 backup files ) # Set log format handler.setFormatter(logging.Formatter( "[%(asctime)s] %(levelname)s in %(module)s: %(message)s" )) # Add handler to Flask's logger app.logger.addHandler(handler) app.logger.setLevel(logging.INFO) * * * ## Catch Unhandled Exceptions Using app.errorhandler(500) can catch unhandled exceptions and log them: ## Example import traceback @app.errorhandler(500) def internal_error(error): # Log the full exception stack app.logger.error("Internal server error:n%s",traceback.format_exc()) # Show a friendly error page to the user return"""500 - Internal Server Error
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