astral-sh/ruff · error

expected function

Error message

expected function

What it means

Scope::expect_function unwraps as_function(): it returns the StmtFunctionDef node only when the scope was introduced by a function (or lambda) statement; any other scope kind panics with 'expected function'. Callers like function.rs use it on body scopes where the kind is known by construction.

Source

Thrown at crates/ty_python_core/src/scope.rs:435

        match self {
            Self::Class(class) => Some(class),
            _ => None,
        }
    }

    pub fn expect_class(&self) -> &AstNodeRef<ast::StmtClassDef> {
        self.as_class().expect("expected class")
    }

    pub fn as_function(&self) -> Option<&AstNodeRef<ast::StmtFunctionDef>> {
        match self {
            Self::Function(function) => Some(function),
            _ => None,
        }
    }

    pub fn expect_function(&self) -> &AstNodeRef<ast::StmtFunctionDef> {
        self.as_function().expect("expected function")
    }

    fn as_type_alias(&self) -> Option<&AstNodeRef<ast::StmtTypeAlias>> {
        match self {
            Self::TypeAlias(type_alias) => Some(type_alias),
            _ => None,
        }
    }

    pub fn expect_type_alias(&self) -> &AstNodeRef<ast::StmtTypeAlias> {
        self.as_type_alias().expect("expected type alias")
    }

    /// Returns the anchor node index for this scope, or `None` for the module scope.
    ///
    /// This is used to compute relative node indices for expressions within the scope,
    /// providing a stable anchor that only changes when the scope-introducing node changes.
    pub fn node_index(&self) -> Option<NodeIndex> {

View on GitHub (pinned to d1087a4b9e)

Solutions

1. Use as_function() and handle None when the scope kind is not guaranteed
2. Match on the scope kind (ScopeKind::Function) before calling expect_*
3. If it fires inside ty on valid Python, minimize the repro and file an issue

Example fix

// before
let fn_node = body_scope.node(db).expect_function(); // panics if not a function scope

// after
let Some(fn_node) = body_scope.node(db).as_function() else { return None; };

if scope.kind() != ScopeKind::Function { return None; } // establish kind before unwrapping

let is_function_scope = |scope: &Scope| matches!(scope.kind(), ScopeKind::Function);

match scope.node().as_function() { Some(func) => { /* ... */ }, None => { /* wrong scope kind: skip or report */ } }

When it happens

Trigger: Calling expect_function on a module, class, comprehension, or type-alias scope - direct misuse, or a lookup path that fetched a different scope than assumed (e.g. a definition whose enclosing scope is not the function body).

Common situations: Contributors writing new scope-based queries without matching on ScopeKind first; semantic-model refactors shifting which scope a node belongs to.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• expected class
• expected type alias
• i is bounded by INLINE_MAX_SEGMENTS
• index is bounded by INLINE_MAX_SEGMENTS
• PySourceType always parses into a module

AI-assisted analysis of astral-sh/ruff@d1087a4b9e (2026-08-20). Data as JSON: /api/errors/d4038e9be15f6994. Report an issue: GitHub.