If I type slots:
class Foo:
__slots__: Tuple[()] = tuple()
Then, in strict mode, mypy (0.812) tells me:
Incompatible types in assignment (expression has type "Tuple[<nothing>, ...]", variable has type "Tuple[]")
I can write:
__slots__: Tuple[()] = cast(Tuple[()], tuple())
But this is ugly. What is the canonical way to do this? What does mypy mean by Tuple[<nothing>, ...]? Tuples are immutable so surely an empty tuple shouldn't be... a variable amount of nothing..?
The problem is not the annotation but the value. Use a literal tuple to unambiguously represent tuples of fixed size, including the empty tuple:
class Foo:
__slots__: Tuple[()] = ()
Note that MyPy will correctly infer the type of this __slots__ even without an annotation.
The callable tuple has a return type of Tuple[T, ...], since for most inputs the output length is not known. The call tuple() is not special cased. As with tuple() there is no value to infer T from, there is no type inhabiting T – its type is <nothing>.