Fix detection of self argument name when positional-only

src/ssort/_method_requirements.py

@@ -44,10 +44,12 @@ def get_method_requirements(node: ast.AST) -> Iterable[str]:
 def _get_method_requirements_for_function_def(
     node: ast.FunctionDef | ast.AsyncFunctionDef,
 ) -> Iterable[str]:
-    if not node.args.args:
+    if node.args.posonlyargs:
+        self_arg = node.args.posonlyargs[0].arg
+    elif node.args.args:
+        self_arg = node.args.args[0].arg
+    else:
         return
 
-    self_arg = node.args.args[0].arg
-
     for statement in node.body:
         yield from _get_attribute_accesses(statement, self_arg)

tests/test_method_requirements.py

@@ -315,3 +315,13 @@ def fun(self):
         """
     )
     assert reqs == ["a", "b"]
+
+
+def test_method_requirements_positional_only_self():
+    reqs = _method_requirements(
+        """
+        def fun(self, /, arg):
+            return self.method(arg)
+        """
+    )
+    assert reqs == ["method"]

tests/test_ssort.py

@@ -653,3 +653,28 @@ def f():
 
     actual = ssort(original)
     assert actual == expected
+
+
+def test_ssort_self_positional_only():
+    original = _clean(
+        """
+        class C:
+            def fun(self, arg, /, notself):
+                return notself._notdep()
+
+            def _notdep(self):
+                pass
+        """
+    )
+    expected = _clean(
+        """
+        class C:
+            def fun(self, arg, /, notself):
+                return notself._notdep()
+
+            def _notdep(self):
+                pass
+        """
+    )
+    actual = ssort(original)
+    assert actual == expected