pallets/flask · error · ValueError

'endpoint' may not contain a dot '.' character.

Error message

'endpoint' may not contain a dot '.' character.

What it means

Blueprint.add_url_rule rejects an endpoint containing '.' because the dot is reserved as the separator between blueprint name and endpoint (final endpoint becomes 'bp.endpoint'). A dot in the user-supplied endpoint would break url_for and theBlueprint prefixing logic. Raises ValueError.

Solutions

1. Remove dots from the endpoint argument: endpoint='list' instead of 'users.list'.
2. Let Flask default the endpoint from the view function's __name__ (ensure that name has no dot, see error 49).
3. Replace separators: endpoint.replace('.', '_').

Example fix

// before
@bp.route('/list', endpoint='users.list')
def list_users(): ...

// after
@bp.route('/list', endpoint='list')
def list_users(): ...

def validate_endpoint(endpoint: str | None) -> str | None:
    if endpoint and '.' in endpoint:
        raise ValueError("endpoint must not contain '.'")
    return endpoint

def endpoint_is_dot_free(endpoint) -> bool:
    return endpoint is None or '.' not in endpoint

When it happens

Trigger: Passing endpoint='users.list' to @bp.route(...); deriving endpoint from a dotted qualified name; copy-pasting an app-level dotted endpoint into a blueprint route.

Common situations: Migrating app routes into a blueprint without renaming endpoints; using fully-qualified function names as endpoints; blueprint factories templating endpoints with dots.

Related errors

• 'name' may not contain a dot '.' character.
• 'view_func' name may not contain a dot '.' character.
• Cannot register a blueprint on itself
• 'name' may not be empty.
• The name ' ' is already registered for blueprint . Use…

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/c95ee4186522c8ee. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/sansio/blueprints.py:428

        extend(self.template_context_processors, app.template_context_processors)

    @setupmethod
    def add_url_rule(
        self,
        rule: str,
        endpoint: str | None = None,
        view_func: ft.RouteCallable | None = None,
        provide_automatic_options: bool | None = None,
        **options: t.Any,
    ) -> None:
        """Register a URL rule with the blueprint. See :meth:`.Flask.add_url_rule` for
        full documentation.

        The URL rule is prefixed with the blueprint's URL prefix. The endpoint name,
        used with :func:`url_for`, is prefixed with the blueprint's name.
        """
        if endpoint and "." in endpoint:
            raise ValueError("'endpoint' may not contain a dot '.' character.")

        if view_func and hasattr(view_func, "__name__") and "." in view_func.__name__:
            raise ValueError("'view_func' name may not contain a dot '.' character.")

        self.record(
            lambda s: s.add_url_rule(
                rule,
                endpoint,
                view_func,
                provide_automatic_options=provide_automatic_options,
                **options,
            )
        )

    @t.overload
    def app_template_filter(self, name: T_template_filter) -> T_template_filter: ...
    @t.overload
    def app_template_filter(

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