Protocol: Supports Less Tham

A PEP 544 protocol to support type hints for functions that depend on items implementing __lt__—any function that relies on sorting, for example.

class _SupportsLessThan(Protocol):
    def __lt__(self, other: Any) -> bool: ...

_T = TypeVar('_T')
_T1 = TypeVar('_T1')
_LT = TypeVar('_LT', bound=_SupportsLessThan)

Example: sorted()

Python 3

@overload
def sorted(__it: Iterable[_LT], *,
           key: None = ...,
           reverse: bool = ...) -> List[_LT]: ...
@overload
def sorted(__it: Iterable[_T], *,
           key: Callable[[_T], _LT] = ...,
           reverse: bool = ...) -> List[_T]: ...

hint	prevents bug
it: Iterable[_T]	it is not iterable
it: Iterable[_LT]	_LT doesn't implement __lt__
key: Optional[Callable[[_T], _LT]]	key is not callable or None
	key is callable with arity ≠ 1
	key returns type that doesn't implement __lt__

Python 2

def sorted(__it: Iterable[_T], *,
           cmp: Optional[Callable[[_T, _T1]], float] = ...,
           key: Optional[Callable[[_T], Any]] = ...,
           reverse: bool = ...) -> List[_T]: ...

hint	prevents bug
it: Iterable[_T]	it is not iterable
cmp: Callable[[_T, _T1], float]	cmp is not callable
	cmp is callable with arity ≠ 2
	cmp does not return float
key: Callable[[_T], Any]]	key is not callable
	key is callable with arity ≠ 1

Example: list.sort()

Python 3

class list(MutableSequence[_T], Generic[_T]):
    ...
    @overload
    def sort(self: List[_LT], *,
             key: None = ...,
             reverse: bool = ...) -> List[_LT]: ...
    @overload
    def sort(self, *,
             key: Callable[[_T], _LT] = ...,
             reverse: bool = ...) -> List[_T]: ...

hint	prevents bug	test
self: List[_LT]	_LT doesn't implement __lt__	test_sort_items_dont_support_less_than_py3
key: Callable[[_T], _LT]]	key is not callable	test_sort_key_not_callable
	key is callable with arity ≠ 1	test_sort_key_wrong_arity
	key returns type that doesn't implement __lt__	test_sort_bad_key_return_py3

Python 2

class list(MutableSequence[_T], Generic[_T]):
    ...
    def sort(self, *,
            cmp: Optional[Callable[[_T, _T1]], float] = ...,
            key: Optional[Callable[[_T], Any]] = ...,
            reverse: bool = ...) -> List[_T]: ...

hint	prevents bug	test
cmp: Optional[Callable[[_T, _T1], float]]	cmp is not callable or None
	cmp is callable with arity ≠ 2	test_sort_list_str_bad_cmp_arity_py2
	cmp does not return float
key: Optional[Callable[[_T], Any]]	key is not callable or None
	key is callable with arity ≠ 1