Tuple & TypeVarTuple Default Arguments

[adam-grant-hendry]

Feature

It would be nice if Tuple and TypeVarTuple supported default arguments (particularly for use with Unpack).

Pitch

As a motivating example, I work with several 3rd-party C++ libraries with python bindings (e.g. PySide6). These bindings often have methods that do not support keyword arguments (C++ doesn't support keyword arguments natively). Sometimes, they have wrappers that forward *args to the particular binding. e.g.

    @staticmethod
    def mouseMove(*args):
        qtpy.QtTest.QTest.mouseMove(*args)

has C++ signatures

void mouseMove(QWindow *window, QPoint pos = QPoint(), int delay = -1)
void mouseMove(QWidget *widget, QPoint pos = QPoint(), int delay = -1)

that has default positional arguments for pos and delay.

Currently, AFAIK, there is no way to specify default arguments to either Tuple or TypeVarTuple. Without specifying default values, @overloads would be required (which one may argue is a sensible alternative, but is tedious when having to be performed for multiple methods in a large codebase):

Argument missing for parameter "args[n]"

Syntax Idea

I'm not sure how it would be implemented, but intuitively I am trying to achieve something like

MouseMoveArgs = tuple[
    qt_api.QtWidgets.QWidget | qt_api.QtGui.QWindow,
    qt_api.QtCore.QPoint = qt_api.QtCore.QPoint(),
    int = -1,  # c int
]

that I can then unpack

class QtBot:
    @staticmethod
    def mouseMove(*args: Unpack[MouseMoveArgs]) -> None: ...

[AlexWaygood]

I'm not sure I fully understand your proposal here, but it sounds like you're proposing a change to the spec for tuple/TypeVarTuple. Discussions like that should happen over at https://github.com/python/typing rather than the mypy issue tracker, since they affect all users of Python typing, not just mypy users.