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[flake8-annotations] Do not automatically set the return type to NoReturn when raising NotImplementedError #18927

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[flake8-annotations] Do not automatically set the return type to NoReturn when raising NotImplementedError #18927

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51 changes: 50 additions & 1 deletion crates/ruff_linter/src/rules/flake8_annotations/helpers.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,8 @@ use ruff_text_size::{TextRange, TextSize};
use crate::Edit;
use crate::checkers::ast::Checker;
use ruff_python_ast::PythonVersion;
use ruff_python_ast::visitor::walk_stmt;
use ruff_python_ast::{ExprCall, ExprName, Stmt};

/// Return the name of the function, if it's overloaded.
pub(crate) fn overloaded_name<'a>(
Expand Down Expand Up @@ -65,8 +67,11 @@ pub(crate) fn auto_return_type(function: &ast::StmtFunctionDef) -> Option<AutoPy
// Determine the terminal behavior (i.e., implicit return, no return, etc.).
let terminal = Terminal::from_function(function);

// If every control flow path raises an exception, return `NoReturn`.
if terminal == Terminal::Raise {
if raises_only_not_implemented_error(function) {
return None;
}

return Some(AutoPythonType::Never);
}

Expand Down Expand Up @@ -227,3 +232,47 @@ fn type_expr(python_type: PythonType) -> Option<Expr> {
PythonType::Generator => None,
}
}

fn raises_only_not_implemented_error(function: &ast::StmtFunctionDef) -> bool {
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@MichaReiser MichaReiser Jun 25, 2025

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It seems unfortunate that we need to traverse the entire function again to determine if all of them were RaiseNotImplemented. Should we add a new Terminal::RaiseNotImplemented variant instead?

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@danparizher danparizher Jun 25, 2025

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Good point—but the extra walk only happens when we’ve already determined the function is Terminal::Raise, so the cost is minimal. Extending Terminal with a RaiseNotImplemented would mix exception-specific semantics into what’s currently a pure control-flow enum and force every existing caller to handle a new case.

struct RaiseVisitor {
only_not_implemented: bool,
seen_any_raise: bool,
}

impl<'a> Visitor<'a> for RaiseVisitor {
fn visit_stmt(&mut self, stmt: &'a Stmt) {
if let Stmt::Raise(ast::StmtRaise { exc, .. }) = stmt {
self.seen_any_raise = true;

let Some(exc) = exc.as_deref() else {
self.only_not_implemented = false;
return;
};

let is_not_impl = match exc {
Expr::Name(ExprName { id, .. }) => id == "NotImplementedError",
Expr::Call(ExprCall { func, .. }) => match func.as_ref() {
Expr::Name(ExprName { id, .. }) => id == "NotImplementedError",
_ => false,
},
_ => false,
};

if !is_not_impl {
self.only_not_implemented = false;
}
} else {
walk_stmt(self, stmt);
}
}
}

let mut visitor = RaiseVisitor {
only_not_implemented: true,
seen_any_raise: false,
};

visitor.visit_body(&function.body);

visitor.seen_any_raise && visitor.only_not_implemented
}
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