Question about Tuple with ellipsis

[mkurnikov]

Does Tuple support form of Tuple[int, str, ...] with more than one parameter before the ellipsis? It seems that it's not, currently.

class TupleMeta(TypingMeta):
    ...
    def __getitem__(self, parameters):
        if self.__tuple_params__ is not None:
            raise TypeError("Cannot re-parameterize %r" % (self,))
        if not isinstance(parameters, tuple):
            parameters = (parameters,)
        if len(parameters) == 2 and parameters[1] == Ellipsis:
            parameters = parameters[:1]
            use_ellipsis = True
            msg = "Tuple[t, ...]: t must be a type."
        else:
            use_ellipsis = False
            msg = "Tuple[t0, t1, ...]: each t must be a type."
        parameters = tuple(_type_check(p, msg) for p in parameters)
        return self.__class__(self.__name__, self.__bases__,
                              dict(self.__dict__), parameters,
                              use_ellipsis=use_ellipsis, _root=True)

I guess, Ellipsis-related check has to be implemented somehow like this:

    if parameters[-1] == Ellipsis:
        parameters = parameters[:-1]
        use_ellipsis = True

[gvanrossum]

PP 484 is clear on the matter; only one type is allowed (and required) t appear before the ellipsis:

• Tuple, used by listing the element types, for example
  Tuple[int, int, str].
  Arbitrary-length homogeneous tuples can be expressed
  using one type and ellipsis, for example Tuple[int, ...].

[srittau]

@AlfreG Your question is probably better suited for our discussion forum: https://github.com/python/typing/discussions

[luckydonald]

@srittau I love that you dug this ticket out after 5 years for that comment.

Careful, even if it reads like that, the non-working Tuple[int, int, str, ...] can't be written as Tuple[int, ...] but you will need Tuple[Any, ...] instead.

[srittau]

I have no idea what happened here. My answer was clearly not meant for this issue, but I'm not sure which issue it was meant for.

[erictraut]

FWIW, if PEP 646 is accepted, it will enable tuple types like this to be expressed as follows:

tuple[int, int, Unpack[tuple[str, ...]]]

(or if the nicer syntax for Unpack is approved)
tuple[int, int, *tuple[str, ...]]

Mypy doesn't yet contain support for PEP 646, but if you're interested in playing around with it, pyright and pyre both implement the latest draft of the PEP.

[JelleZijlstra]

The nicer syntax was approved but will only be in Python 3.11. We'll backport Unpack support to older versions in typing-extensions.

[erictraut]

That's great news about the nicer syntax being approved. Do you recall where you saw that? I'm interested in the details. I presume that the more general syntax change proposed in PEP 637 was not accepted.

[JelleZijlstra]

python/steering-council#59 (comment)