Optional key to itertools.groupby

[achampion]

itertools.groupby has an @overload type signature but this seems makes Optional[Callable[[T], S]] not match, e.g.:

def fn(iterable: Iterable[T], key: Optional[Callable[[T], S]] = None):
    iterator: Iterator[Tuple[S, Iterator[T]]] = itertools.groupby(iterable, key)

Fails with:

error: Argument 2 to "groupby" has incompatible type "Optional[Callable[[T], S]]"; expected "Callable[[T], S]"

The revealed type of itertools.groupby:

Overload(def [_T] (iterable: typing.Iterable[_T`-1]) -> typing.Iterator[Tuple[_T`-1, typing.Iterator[_T`-1]]], def [_T, _S] (iterable: typing.Iterable[_T`-1], key: def (_T`-1) -> _S`-2) -> typing.Iterator[Tuple[_S`-2, typing.Iterator[_T`-1]]])

It is legitimate to explicitly pass None as key, so is this expected behaviour?
You can do a similar @overload for myfunc to overcome the error.

Versions:
Python 3.7.2
MyPy 0.670

[ilevkivskyi]

I just tried and groupby([1, 2, 3, 3, 3], None) indeed doesn't crash, but according to typeshed this call is illegal. You can open an issue there, if you are sure this is actually intended/documented behavior, or you can also try something like this:

if key is not None:
    iterator = itertools.groupby(iterable, key)
else:
    ...

[achampion]

Thanks will do. Wasn't sure if this was a mypy issue or a typeshed issue.
With the @overloads for itertools.groupby(), should Optional[Callable[[T], S]] match?

While you can do if key is not None: the problem with that is you can't effectively describe the type of iterator, e.g.:

iterator: Iterator[Tuple[S, Iterator[T]]]
if key is not None:
    iterator = itertools.groupby(iterable, key)
else:
    iterator = itertools.groupby(iterable)  # Fail

You get the error:

error: Incompatible types in assignment (expression has type "Iterator[Tuple[T, Iterator[T]]]", variable has type "Iterator[Tuple[S, Iterator[T]]]")

And you can't declare the types in each branch because you will get a redeclaration error Name 'iterator' already defined on line ...

[ilevkivskyi]

With the @overloads for itertools.groupby(), should Optional[Callable[[T], S]] match?

No, with current definition in typeshed it is illegal, Optional means it can be None (not that it is an optional argument), while with current definition neither of two overloads accepts None.