Debugging Tornado applications

Tornado is a powerful Python web framework and asynchronous networking library. It's designed to handle thousands of simultaneous connections, making it an excellent choice for real-time web applications. However, as with any software development, debugging is an essential part of the process. This article aims to introduce beginners to debugging Tornado applications.

Understanding Tornado Debugging

Tornado provides a debug mode that can be very helpful during development. When activated, it provides more detailed error pages when an uncaught exception arises in a request. It also allows the autoreload feature, which restarts the server whenever a tracked file changes. To enable it, set the debug attribute in application settings to True.

tornado.web.Application(handlers, debug=True)

Note: Be aware, it's not recommended to use debug mode in a production environment due to potential security risks.

Using Logging

Proper logging can be your best friend while debugging. Tornado uses Python’s built-in logging library. To write a log message from your code, you can call the logging module’s methods like logging.info(), logging.warning(), logging.error(), etc.

import logging
logging.error('Something went wrong!')

Exception Handling

Tornado's RequestHandler has a special method called write_error(), which is designed to handle any uncaught exceptions. You can override this method in your request handler to customize error handling.

def write_error(self, status_code, **kwargs):
    self.write("Gosh darnit, user! You caused a %d error." % status_code)

Using Debugging Tools

Python provides a built-in debugger called pdb. However, there's a more powerful debugger called pdb++, which is a drop-in replacement improving pdb's UI and adding syntax highlighting, sticky mode, etc. To use it, you just need to install it with pip, and it will be used automatically when you invoke pdb.

pip install pdbpp

To use pdb or pdb++, you just need to add the following line to your code:

import pdb; pdb.set_trace()

This line will start the debugger, and the execution will stop at this line. You can then inspect variables, execute statements, and control the execution flow.

Unit Testing

Tornado provides a tornado.testing module, which includes some test classes that simplify asynchronous testing. The AsyncHTTPTestCase is the base class for tests of Tornado applications using real HTTP client/server communication.

from tornado.testing import AsyncHTTPTestCase
from myapp import MyApplication

class MyTest(AsyncHTTPTestCase):
    def get_app(self):
        return MyApplication()
    
    def test_homepage(self):
        response = self.fetch('/')
        self.assertEqual(response.code, 200)

This example tests that a GET request to the path / returns an HTTP 200 response code.

Conclusion

Debugging is a critical part of the development process, and understanding how to effectively debug your Tornado applications will make you a more efficient and effective developer. Use the debug mode wisely, understand how to use logs, learn exception handling, know the tools at your disposal, and always write proper tests.

Remember: Debugging is not just about fixing bugs. It's also about understanding how your code behaves and ensuring it works as expected.