functools: add TopologicalSorter for py39 (#4068)

* functools: add TopologicalSorter for py39

* fix flake8 error

Co-authored-by: hauntsaninja <>

stdlib/3/functools.pyi

@@ -97,3 +97,15 @@ if sys.version_info >= (3, 8):
         @overload
         def __get__(self, instance: _S, owner: Optional[Type[Any]] = ...) -> _T: ...
         def __set_name__(self, owner: Type[Any], name: str) -> None: ...
+
+if sys.version_info >= (3, 9):
+    class TopologicalSorter(Generic[_T]):
+        def __init__(self, graph: Optional[Dict[_T, Iterable[_T]]] = ...) -> None: ...
+        def add(self, node: _T, *predecessors: _T) -> None: ...
+        def prepare(self) -> None: ...
+        def is_active(self) -> bool: ...
+        def done(self, *nodes: _T) -> None: ...
+        def get_ready(self) -> Tuple[_T, ...]: ...
+        def static_order(self) -> Iterable[_T]: ...
+
+    class CycleError(ValueError): ...