locustio/locust · error · AttributeError

The login_manager is only available with --web-login.

Error message

The login_manager is only available with --web-login.

What it means

The WebUI exposes login_manager as a property, but it only returns the internal Flask-Login manager when the web UI was started with --web-login. Accessing it otherwise raises an AttributeError instructing you to enable web login. This is an intentional guard, not a missing attribute.

Source

Thrown at locust/web.py:618

            return {}, 201

        @app_blueprint.route("/worker-count")
        @self.auth_required_if_enabled
        def get_worker_count():
            return {
                "worker_count": self.environment.runner.worker_count
                if isinstance(self.environment.runner, MasterRunner)
                else 0
            }

        app.register_blueprint(app_blueprint)

    @property
    def login_manager(self):
        if self.web_login:
            return self._login_manager

        raise AttributeError("The login_manager is only available with --web-login.\n")

    @login_manager.setter
    def login_manager(self, value):
        self._login_manager = value

    def start(self):
        self.greenlet = gevent.spawn(self.start_server)
        self.greenlet.link_exception(greenlet_exception_handler)

    def start_server(self):
        if self.tls_cert and self.tls_key:
            self.server = pywsgi.WSGIServer(
                (self.host, self.port), self.app, log=None, keyfile=self.tls_key, certfile=self.tls_cert
            )
        else:

            class RewriteFilter(logging.Filter):
                def filter(self, record) -> bool:

View on GitHub (pinned to f391a716e1)

Solutions

1. Start locust with --web-login (and configure authentication, e.g. --web-login with the auth env settings)
2. Guard access: check `web_ui.web_login` before touching login_manager
3. Override or register Flask-Login differently in a custom subclass where you manage the manager yourself

Example fix

# before
web_ui.login_manager.login_view = 'login'  # AttributeError without --web-login

# after
if web_ui.web_login:
    web_ui.login_manager.login_view = 'login'

if getattr(web_ui, 'web_login', False):
    lm = web_ui.login_manager

def has_login_manager(web_ui) -> bool:
    return bool(getattr(web_ui, 'web_login', False)) and getattr(web_ui, '_login_manager', None) is not None

try:
    lm = web_ui.login_manager
except AttributeError:
    lm = None  # --web-login not enabled; skip auth-related setup
if lm:
    lm.login_view = 'login'

When it happens

Trigger: Custom web UI extensions or plugins touching `environment.web_ui.login_manager` (or subclass code) when locust was launched without --web-login; code reading/flashing login state in a custom route.

Common situations: Deploying the same web UI extension across environments where only some enable --web-login; upgrading locust and adopting the login feature without adding the flag.

AI-assisted analysis of locustio/locust@f391a716e1 (2026-08-29). Data as JSON: /api/errors/64195707fe6d7439. Report an issue: GitHub.