astral-sh/ruff · error

Will return Some() when called on class literal

Error message

Will return Some() when called on class literal

What it means

`typed_dict_fallback_class_member` resolves a member on the TypedDict fallback class literal (from typing or typing_extensions, depending on `module`). The stated invariant: an MRO member lookup on a class literal always returns Some because the MRO terminates in `object`. The expect fires when the loaded typeshed does not contain the requested member anywhere in the fallback class's MRO.

Source

Thrown at crates/ty_python_semantic/src/types/class/typed_dict.rs:1086

    )
}

pub(super) fn typed_dict_fallback_class_member<'db>(
    db: &'db dyn Db,
    env: &ProgramEnvironment<'db>,
    module: TypingModule,
    lookup_policy: MemberLookupPolicy,
    name: &str,
) -> PlaceAndQualifiers<'db> {
    let fallback = match module {
        TypingModule::Typing => KnownClass::TypedDictFallback,
        TypingModule::TypingExtensions => KnownClass::ExtensionTypedDictFallback,
    };

    fallback
        .to_class_literal(db, env)
        .find_name_in_mro_with_policy(db, env, name, lookup_policy)
        .expect("Will return Some() when called on class literal")
}

pub(super) fn typed_dict_class_member<'db>(
    db: &'db dyn Db,
    env: &ProgramEnvironment<'db>,
    class: ClassType<'db>,
    module: TypingModule,
    lookup_policy: MemberLookupPolicy,
    name: &str,
) -> PlaceAndQualifiers<'db> {
    let self_class = class.class_literal(db);

    typed_dict_inherited_class_member(
        db,
        env,
        TypedDictType::new(class),
        module,
        lookup_policy,

View on GitHub (pinned to d1087a4b9e)

Solutions

1. Run ty with the bundled vendored typeshed instead of a custom one.
2. Update ty so its member requests match the typeshed version it vendors.
3. If a custom typeshed is mandatory, sync it to the version ty's snapshot is pinned to and verify the missing member exists in the stub.
4. Report the member name (visible in the panic backtrace) upstream if the bundled setup panics.

Example fix

# before: fallback stub missing the member ty looks up
# <typeshed>/stdlib/typing.pyi: class TypedDict(metaclass=_TypedDictMeta): ...

# after: restore/patch the stub so the member resolves, e.g.
# class TypedDict(metaclass=_TypedDictMeta):
#     def __call__(self, *args, **kwargs) -> Any: ...

# verify the member exists in the stubs ty will load (substitute the member
# from your panic backtrace for __call__):
grep -rn 'def __call__' <typeshed>/stdlib/typing.pyi <typeshed>/stdlib/builtins.pyi
# no hits anywhere in the fallback MRO means this panic is expected

When it happens

Trigger: Member access on a TypedDict-shaped type that falls back to the stub class while the active typeshed's `typing.pyi`/`typing_extensions` fallback stubs lack the member (and `object` does not provide it either).

Common situations: Custom, stripped, or outdated typeshed directories; mixing a typeshed snapshot with a ty binary from a different era; typeshed API changes where a member moved or was renamed while ty still requests the old name.

Related errors

• `find_name_in_mro` should return `Some` for a class literal
• `object` should always be a non-generic class in typeshed
• TypedDict code generation should use a TypedDict instance
• TypedDictParams should be available for CodeGeneratorKind::T
• Expected `NamedTuple` definition to be an assignment

AI-assisted analysis of astral-sh/ruff@d1087a4b9e (2026-08-20). Data as JSON: /api/errors/37c84e56f3e8bf35. Report an issue: GitHub.