Print raw HTTP request in Flask or WSGI

Question

I am debugging a microcontroller I've built which is writing raw HTTP requests line by line. I am using Flask for my backend and I would like to see the entire request as it appears in this format:

GET / HTTP/1.1
Content-length: 123
User-agent: blah
...

I know Flask is based on WSGI. Is there anyway to get this to work with Flask?

Answer 1

Yes, Flask is a WSGI application, so it is trivial to wrap your app in an extra layer that logs the request:

import pprint

class LoggingMiddleware(object):
    def __init__(self, app):
        self._app = app

    def __call__(self, env, resp):
        errorlog = env['wsgi.errors']
        pprint.pprint(('REQUEST', env), stream=errorlog)

        def log_response(status, headers, *args):
            pprint.pprint(('RESPONSE', status, headers), stream=errorlog)
            return resp(status, headers, *args)

        return self._app(env, log_response)

This defines a piece of middleware to wrap your Flask application in. The advantage is that it operates entirely independent of Flask, giving you unfiltered insight into what goes in and what comes out.

How you apply the middleware depends on the exact WSGI server you are using; see your WSGI server documentation.

When running Flask with the built-in server (app.run()), do:

if __name__ == '__main__':
    app.wsgi_app = LoggingMiddleware(app.wsgi_app)
    app.run()

The little app.wsgi_app wrapping dance places the LoggingMiddleware around the Flask WSGI application.

The output goes to the wsgi.error stream; where that ends up again depends on your WSGI server; mod_wsgi puts this in the Apache error log for your site, the bundled Flask server prints this to stderr.