Edited:
I was looking at the type annotations of the built-in zip()
function.
I understand that overload (in the context of type checking), can modify the behaviour of a function depending on the type of parameters it's given.
@overload
def zip(__iter1: Iterable[_T1]) -> List[Tuple[_T1]]: ...
@overload
def zip(__iter1: Iterable[_T1],
__iter2: Iterable[_T2]) -> List[Tuple[_T1, _T2]]: ...
@overload
def zip(__iter1: Iterable[_T1], __iter2: Iterable[_T2],
__iter3: Iterable[_T3]) -> List[Tuple[_T1, _T2, _T3]]: ...
@overload
def zip(__iter1: Iterable[_T1], __iter2: Iterable[_T2], __iter3: Iterable[_T3],
__iter4: Iterable[_T4]) -> List[Tuple[_T1, _T2, _T3, _T4]]: ...
@overload
def zip(__iter1: Iterable[_T1], __iter2: Iterable[_T2], __iter3: Iterable[_T3],
__iter4: Iterable[_T4], __iter5: Iterable[_T5]) -> List[Tuple[_T1, _T2, _T3, _T4, _T5]]: ...
@overload
def zip(__iter1: Iterable[Any], __iter2: Iterable[Any], __iter3: Iterable[Any],
__iter4: Iterable[Any], __iter5: Iterable[Any], __iter6: Iterable[Any],
*iterables: Iterable[Any]) -> List[Tuple[Any, ...]]: ...
In this example, however, the type is the same (Any). So what purpose does overload serve?
Why not just use the last function, which takes in an arbitrary number of parameters. Why create the first five which essentially say: if there's one argument do this, if there's two do that, if there's 3 ... This seems to violate the DRY principle.
@overload
def zip(__iter1: Iterable[Any], __iter2: Iterable[Any], __iter3: Iterable[Any],
__iter4: Iterable[Any], __iter5: Iterable[Any], __iter6: Iterable[Any],
*iterables: Iterable[Any]) -> List[Tuple[Any, ...]]: ...
The overload
decorator is for static type analysis, not for implementation. In fact, the code shown in the question are just the type annotations - zip
is a builtin, it is not implemented in Python.
The purpose of the various overloads is to preserve the number and type of arguments. For example, it states that a zip
over three iterables yields tuples with three elements matching the type of the iterable elements. The final overload with variadic *args
and Any
is merely a catch-all for unspecified cases.