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

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Protocol: Supports Less Tham

Example: sorted()

Python 3

Python 2

Example: list.sort()

Python 3

Python 2