Implement exception-based catchall host routing with flask & tests

host_routed_app.py

from werkzeug.routing import Rule as BaseRule


class Rule(BaseRule):
    except_hosts = []

    def match(self, path):
        """ Monkey patched version of the default Werkzeug behavior to achieve host matching with catchall.

        By default, if host matching is enabled, the match is only done if the host is always provided, which is
        unpractical. This modified version matches normally if host matching is not enabled. If host matching is
        enabled, if it fails, and if the host is not part of a blacklist, it will try matching again without host
        matching ('catch all').
        """
        host, route = path.split('|')
        result = super().match(path)
        if result is None and host and host not in self.except_hosts:
            result = super().match('|' + route)
        return result


def configure_app_host_routing(app, except_hosts):
    app.url_rule_class = type('LocalRule', (Rule, ), {'except_hosts': except_hosts})
    app.url_map.host_matching = True


### EXAMPLE

from flask import Flask

app = Flask('test_app')
configure_app_host_routing(app, ['specificdomain.com'])

# add host-specific endpoint
@app.route('/<argument>', host='specificdomain.com')
def specific_host_endpoint():
    return

# add normal endpoint, could be a blueprint too.
@app.route('/test/<id>')
def test():
    return


### TEST

import unittest

from werkzeug.exceptions import NotFound


class TestHostRouting(unittest.TestCase):
    def main_test(self):        
        m = app.url_map

        # host specific endpoint is routed correctly for specificdomain.com
        self.assertEqual(
            ('specific_host_endpoint', {'argument': 'yo'}),
            m.bind('specificdomain.com').match('/yo'),
        )
        with self.assertRaises(NotFound):
            m.bind('specificdomain.com').match('/yo/')

        # host specific endpoint does not work for other domains
        with self.assertRaises(NotFound):
            m.bind('domain.com').match('/yo')
        with self.assertRaises(NotFound):
            m.bind('anything.specificdomain.com').match('/yo')

        # normal endpoint works as expected
        self.assertEqual(('test', {'id': '1'}), m.bind('domain.com').match('/test/1'))
        self.assertEqual(('test', {'id': '1'}), m.bind('anything.domain.com').match('/test/1'))
        # normal endpoint doesn't work for specific host
        with self.assertRaises(NotFound):
            m.bind('specificdomain.com').match('/test/1')

TestHostRouting('main_test').main_test()