Flask · Lesson 8 of 15

Error Handlers and Logging

Catch 404/500 consistently and log with a request id.

Intermediate
14 min read
3 objectives

Before this lessonLesson 7: JSON APIs

What you will learn

Register error handlers
Log exceptions
Add a request id

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Users should see a clean error page. You should see a stack trace with a request id in the logs.

Handlers

import logging
from flask import render_template, jsonify, g

log = logging.getLogger(__name__)

@app.errorhandler(404)
def not_found(e):
    if request.path.startswith("/api/"):
        return jsonify(error="Not found"), 404
    return render_template("404.html"), 404

@app.errorhandler(500)
def server_error(e):
    log.exception("unhandled error request_id=%s", g.get("request_id"))
    if request.path.startswith("/api/"):
        return jsonify(error="Internal Server Error"), 500
    return render_template("500.html"), 500

Request id middleware

import uuid

@app.before_request
def stamp_id():
    g.request_id = request.headers.get("X-Request-Id", uuid.uuid4().hex)

@app.after_request
def set_id(response):
    response.headers["X-Request-Id"] = g.request_id
    return response

Configure logging once in the factory: structured JSON in production, pretty traces in development.

Up next · Lesson 9File Uploads and Static MediaAccept files safely, store them outside the code tree and serve them in development.