Add support for tuple slices.

[jhance]

Apparently python happily accepts any slice, even if its
out of range (and just gives an empty tuple). Personally,
I find that behavior weird, but I opted to not check any
ranges because python itself doesn't care.
#886

[o11c]

Looks like you're missing Tuple[T, ...].

[o11c]

Homogenous tuples (of either kind) should support non-literal slices.

[jhance]

No they shouldn't, you lose the length of the tuple if you don't have a literal slice. I can see an argument for them resolving to variable-length tuples, but I dislike it.

I don't mind having homogenous tuples work with arbitrary slices, though.

I'll probably get homogenous tuples working tomorrow, I thought it would be simple but for some reason its treating it like not-a-tuple...

[o11c]

It was really screwy to me at first, how the type shows up internally as builtins.tuple[T], not typing.Tuple[T, ...].

[JukkaL]

Cool, thanks for the PR!

Slicing of Tuple[t, ...] values already works. They are internally regular classes and support methods defined in buildins.tuple, including __getitem__ with a slice argument.

Slicing homogeneous, fixed-length tuples like Tuple[int, int, int] with non-literal slices could be valid. Similarly, we should probably support indexing them with non-literal integers (e.g., t[n]).

One another thing would be nice to support: negative literal int in slice (e.g., t[:-1]).

[jhance]

Slicing of Tuple[t, ...] values already works. They are internally regular classes and support methods defined in buildins.tuple, including getitem with a slice argument.

FYI I was seeing mypy crash failing to locate the name slice when I so much as added a slice of Tuple[T, ...] to the test file.

[o11c]

That's because of #890