[flake8-self] Ignore attribute accesses on instance-like variables (SLF001) (#16149)

crates/ruff_python_ast/src/helpers.rs

@@ -548,6 +548,13 @@ pub fn is_dunder(id: &str) -> bool {
     id.starts_with("__") && id.ends_with("__")
 }
 
+/// Whether a name starts and ends with a single underscore.
+///
+/// `_a__` is considered neither a dunder nor a sunder name.
+pub fn is_sunder(id: &str) -> bool {
+    id.starts_with('_') && id.ends_with('_') && !id.starts_with("__") && !id.ends_with("__")
+}
+
 /// Return `true` if the [`Stmt`] is an assignment to a dunder (like `__all__`).
 pub fn is_assignment_to_a_dunder(stmt: &Stmt) -> bool {
     // Check whether it's an assignment to a dunder, with or without a type