modify flask url before routing

Question

My Flask app has url routing defined as

self.add_url_rule('/api/1/accounts/<id_>', view_func=self.accounts, methods=['GET'])

Problem is one of the application making queries to this app adds additional / in url like /api/1//accounts/id. It's not in my control to correct the application which makes such queries so I cant change it.

To resolve this problem currently I have added multiple rules

self.add_url_rule('/api/1/accounts/<id_>', view_func=self.accounts, methods=['GET'])
self.add_url_rule('/api/1//accounts/<id_>', view_func=self.accounts, methods=['GET'])

There are number of such routes and it's ugly workaround. Is there a way in flask to modify URL before it hits the routing logic?

Answer 1

I'd normalise the path before it gets to Flask, either by having the HTTP server that hosts the WSGI container or a proxy server that sits before your stack do this, or by using WSGI middleware.

The latter is easily written:

import re
from functools import partial


class PathNormaliser(object):
    _collapse_slashes = partial(re.compile(r'/+').sub, r'/')

    def __init__(self, application):
        self.application = application

    def __call__(self, env, start_response):    
        env['PATH_INFO'] = self._collapse_slashes(env['PATH_INFO'])                                                                                                  
        return self.application(env, start_response)

You may want to log that you are applying this transformation, together with diagnostic information like the REMOTE_HOST and HTTP_USER_AGENT entries. Personally, I'd force that specific application to generate non-broken URLs as soon as possible.

Look at your WSGI server documentation to see how to add in extra WSGI middleware components.