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Type variable tuple variance #741

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df6ef1f
add bound/variance parameters to `TypeVarTuple`
KotlinIsland Apr 23, 2026
7a6e221
fixup! add bound/variance parameters to `TypeVarTuple`
KotlinIsland Apr 23, 2026
550f1d1
fixup! add bound/variance parameters to `TypeVarTuple`
KotlinIsland Apr 24, 2026
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1 change: 1 addition & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
Python 3.9. The `typing` implementation has always raised an error, and the
`typing_extensions` implementation has raised an error on Python 3.10+ since
`typing_extensions` v4.6.0. Patch by Brian Schubert.
- Add `bound` and variance parameters to `TypeVarTuple`.

# Release 4.15.0 (August 25, 2025)

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54 changes: 49 additions & 5 deletions src/test_typing_extensions.py
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Expand Up @@ -1756,11 +1756,9 @@ def test_annotation_and_optional_default(self):
annotation : annotation,
Optional[int] : Optional[int],
Optional[List[str]] : Optional[List[str]],
Optional[annotation] : Optional[annotation],
Optional[annotation] : Optional[annotation],
Union[str, None, str] : Optional[str],
Unpack[Tuple[int, None]]: Unpack[Tuple[int, None]],
# Note: A starred *Ts will use typing.Unpack in 3.11+ see Issue #485
Unpack[Ts] : Unpack[Ts],
}
# contains a ForwardRef, TypeVar(~prefix) or no expression
do_not_stringify_cases = {
Expand All @@ -1776,6 +1774,8 @@ def test_annotation_and_optional_default(self):
Union[str, "Union[None, StrAlias]"]: Optional[str],
Union["annotation", T_default] : Union[annotation, T_default],
Annotated["annotation", "nested"] : Annotated[Union[int, None], "data", "nested"],
# Note: A starred *Ts will use typing.Unpack in 3.11+ see Issue #485
Unpack[Ts] : Unpack[Ts],
}
if TYPING_3_10_0: # cannot construct UnionTypes before 3.10
do_not_stringify_cases["str | NoneAlias | StrAlias"] = str | None
Expand Down Expand Up @@ -6552,7 +6552,10 @@ def test_basic_plain(self):

def test_repr(self):
Ts = TypeVarTuple('Ts')
self.assertEqual(repr(Unpack[Ts]), f'{Unpack.__module__}.Unpack[Ts]')
if not hasattr(typing, 'TypeVarTuple') or sys.version_info >= (3, 15):
self.assertEqual(repr(Unpack[Ts]), f'{Unpack.__module__}.Unpack[~Ts]')
else:
self.assertEqual(repr(Unpack[Ts]), f'{Unpack.__module__}.Unpack[Ts]')

def test_cannot_subclass_vars(self):
with self.assertRaises(TypeError):
Expand Down Expand Up @@ -6750,7 +6753,44 @@ def test_basic_plain(self):

def test_repr(self):
Ts = TypeVarTuple('Ts')
self.assertEqual(repr(Ts), 'Ts')
Ts_co = TypeVarTuple('Ts_co', covariant=True)
Ts_contra = TypeVarTuple('Ts_contra', contravariant=True)
Ts_infer = TypeVarTuple('Ts_infer', infer_variance=True)
Ts_2 = TypeVarTuple('Ts_2')
if not hasattr(typing, 'TypeVarTuple') or sys.version_info >= (3, 15):
self.assertEqual(repr(Ts), '~Ts')
self.assertEqual(repr(Ts_2), '~Ts_2')

self.assertEqual(repr(Ts_co), '+Ts_co')
self.assertEqual(repr(Ts_contra), '-Ts_contra')
self.assertEqual(repr(Ts_infer), 'Ts_infer')
else:
# On other versions we use typing.TypeVarTuple, but it is not aware of
# variance. Not worth creating our own version of TypeVarTuple
# for this.
self.assertEqual(repr(Ts), 'Ts')
self.assertEqual(repr(Ts_2), 'Ts_2')

self.assertEqual(repr(Ts_co), 'Ts_co')
self.assertEqual(repr(Ts_contra), 'Ts_contra')
self.assertEqual(repr(Ts_infer), 'Ts_infer')

def test_variance(self):
Ts_co = TypeVarTuple('Ts_co', covariant=True)
Ts_contra = TypeVarTuple('Ts_contra', contravariant=True)
Ts_infer = TypeVarTuple('Ts_infer', infer_variance=True)

self.assertIs(Ts_co.__covariant__, True)
self.assertIs(Ts_co.__contravariant__, False)
self.assertIs(Ts_co.__infer_variance__, False)

self.assertIs(Ts_contra.__covariant__, False)
self.assertIs(Ts_contra.__contravariant__, True)
self.assertIs(Ts_contra.__infer_variance__, False)

self.assertIs(Ts_infer.__covariant__, False)
self.assertIs(Ts_infer.__contravariant__, False)
self.assertIs(Ts_infer.__infer_variance__, True)

def test_no_redefinition(self):
self.assertNotEqual(TypeVarTuple('Ts'), TypeVarTuple('Ts'))
Expand Down Expand Up @@ -7076,6 +7116,10 @@ def test_typing_extensions_defers_when_possible(self):
exclude |= {
'TypeAliasType'
}
if sys.version_info < (3, 15):
exclude |= {
'TypeVarTuple'
}
if not typing_extensions._PEP_728_IMPLEMENTED:
exclude |= {'TypedDict', 'is_typeddict'}
for item in typing_extensions.__all__:
Expand Down
50 changes: 41 additions & 9 deletions src/typing_extensions.py
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Expand Up @@ -1671,7 +1671,10 @@ def TypeAlias(self, parameters):

def _set_default(type_param, default):
type_param.has_default = lambda: default is not NoDefault
type_param.__default__ = default
if default is NoDefault:
type_param.__default__ = default
else:
type_param.__default__ = typing._type_check(default, "Default must be a type.")


def _set_module(typevarlike):
Expand Down Expand Up @@ -1824,7 +1827,7 @@ def __new__(cls, name, *, bound=None,
paramspec = typing.ParamSpec(name, bound=bound,
covariant=covariant,
contravariant=contravariant)
paramspec.__infer_variance__ = infer_variance
paramspec.__infer_variance__ = bool(infer_variance)

_set_default(paramspec, default)
_set_module(paramspec)
Expand Down Expand Up @@ -2571,20 +2574,33 @@ def _unpack_args(*args):
return newargs


if _PEP_696_IMPLEMENTED:
if sys.version_info >= (3, 15):
from typing import TypeVarTuple

elif hasattr(typing, "TypeVarTuple"): # 3.11+

# Add default parameter - PEP 696
# Add default parameter - PEP 696 and bound/variance parameters
class TypeVarTuple(metaclass=_TypeVarLikeMeta):
"""Type variable tuple."""

_backported_typevarlike = typing.TypeVarTuple

def __new__(cls, name, *, default=NoDefault):
tvt = typing.TypeVarTuple(name)
_set_default(tvt, default)
def __new__(cls, name, *, bound=None,
covariant=False, contravariant=False,
infer_variance=False, default=NoDefault):

if _PEP_696_IMPLEMENTED:
# can pass default argument
tvt = typing.TypeVarTuple(name, default=default)
else:
tvt = typing.TypeVarTuple(name)
_set_default(tvt, default)

tvt.__bound__ = typing._type_check(bound, "Bound must be a type.")
tvt.__covariant__ = bool(covariant)
tvt.__contravariant__ = bool(contravariant)
tvt.__infer_variance__ = bool(infer_variance)

_set_module(tvt)

def _typevartuple_prepare_subst(alias, args):
Expand Down Expand Up @@ -2689,8 +2705,16 @@ def get_shape(self) -> Tuple[*Ts]:
def __iter__(self):
yield self.__unpacked__

def __init__(self, name, *, default=NoDefault):
def __init__(self, name, *, bound=None, covariant=False, contravariant=False,
infer_variance=False, default=NoDefault):
self.__name__ = name
self.__covariant__ = bool(covariant)
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@JelleZijlstra JelleZijlstra Apr 23, 2026

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Why does this branch call bool() on the variance-related arguments and _type_check on bound and the above one doesn't? We should have things behave the same way across versions.

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@KotlinIsland KotlinIsland Apr 24, 2026 •
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the implementation of ParamSpec also has this discrepancy, i will update it accordingly

should _set_default also invoke _type_check?

self.__contravariant__ = bool(contravariant)
self.__infer_variance__ = bool(infer_variance)
if bound:
self.__bound__ = typing._type_check(bound, 'Bound must be a type.')
else:
self.__bound__ = None
_DefaultMixin.__init__(self, default)

# for pickling:
Expand All @@ -2701,7 +2725,15 @@ def __init__(self, name, *, default=NoDefault):
self.__unpacked__ = Unpack[self]

def __repr__(self):
return self.__name__
if self.__infer_variance__:
prefix = ''
elif self.__covariant__:
prefix = '+'
elif self.__contravariant__:
prefix = '-'
else:
prefix = '~'
return prefix + self.__name__

def __hash__(self):
return object.__hash__(self)
Expand Down
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