Skip to content

gh-144361: Fix NameError when type parameter default refers to a forward name #152208

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
tangyuan0821 wants to merge 2 commits into
base: main
Choose a base branch
from
+70 −3
Open

gh-144361: Fix NameError when type parameter default refers to a forward name #152208

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
201b532
gh-144361: Fix NameError when type parameter default refers to a forw…
tangyuan0821 Jun 25, 2026
2a0ffac
gh-144361: Add tests for forward reference default in type parameters
tangyuan0821 Jun 26, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
48 changes: 48 additions & 0 deletions Lib/test/test_typing.py
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -823,6 +823,54 @@ def test_pickle(self):
self.assertEqual(z.__bound__, typevar.__bound__)
self.assertEqual(z.__default__, typevar.__default__)

def test_forward_reference_default_typevar(self):
ns = run_code(
"""
class A[T, U = ForwardName]:
pass
"""
)
U, A = ns["A"].__type_params__[1], ns["A"]
with self.assertRaises(NameError):
U.__default__
result = A[int]
self.assertIsInstance(result.__args__[0], type)
self.assertIs(int, result.__args__[0])
self.assertIsInstance(result.__args__[1], ForwardRef)
self.assertEqual(result.__args__[1].__forward_arg__, 'ForwardName')

def test_forward_reference_default_typevartuple(self):
ns = run_code(
"""
class A[T, *Ts = ForwardName]:
pass
"""
)
Ts, A = ns["A"].__type_params__[1], ns["A"]
with self.assertRaises(NameError):
Ts.__default__
result = A[int]
self.assertIsInstance(result.__args__[0], type)
self.assertIs(int, result.__args__[0])
self.assertIsInstance(result.__args__[1], ForwardRef)
self.assertEqual(result.__args__[1].__forward_arg__, 'ForwardName')

def test_forward_reference_default_paramspec(self):
ns = run_code(
"""
class A[T, **P = ForwardName]:
pass
"""
)
P, A = ns["A"].__type_params__[1], ns["A"]
with self.assertRaises(NameError):
P.__default__
result = A[int]
self.assertIsInstance(result.__args__[0], type)
self.assertIs(int, result.__args__[0])
self.assertIsInstance(result.__args__[1], ForwardRef)
self.assertEqual(result.__args__[1].__forward_arg__, 'ForwardName')


def template_replace(templates: list[str], replacements: dict[str, list[str]]) -> list[tuple[str]]:
"""Renders templates with possible combinations of replacements.
Expand Down
21 changes: 18 additions & 3 deletions Lib/typing.py
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1099,7 +1099,12 @@ def _typevartuple_prepare_subst(self, alias, args):
raise TypeError(f"Too few arguments for {alias};"
f" actual {alen}, expected at least {plen-1}")
if left == alen - right and self.has_default():
replacement = _unpack_args(self.__default__)
try:
default = self.__default__
except NameError:
default = annotationlib.call_evaluate_function(
self.evaluate_default, annotationlib.Format.FORWARDREF)
replacement = _unpack_args(default)
else:
replacement = args[left: alen - right]

Expand All @@ -1125,7 +1130,12 @@ def _paramspec_prepare_subst(self, alias, args):
params = alias.__parameters__
i = params.index(self)
if i == len(args) and self.has_default():
args = (*args, self.__default__)
try:
default = self.__default__
except NameError:
default = annotationlib.call_evaluate_function(
self.evaluate_default, annotationlib.Format.FORWARDREF)
args = (*args, default)
if i >= len(args):
raise TypeError(f"Too few arguments for {alias}")
# Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
Expand Down Expand Up @@ -1185,7 +1195,12 @@ def _generic_class_getitem(cls, args):
for param in parameters:
prepare = getattr(param, '__typing_prepare_subst__', None)
if prepare is not None:
args = prepare(cls, args)
try:
args = prepare(cls, args)
except NameError:

@JelleZijlstra JelleZijlstra Jun 25, 2026

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I don't understand this part, why do we have to add default here?

@tangyuan0821 tangyuan0821 Jun 26, 2026

Copy link
Copy Markdown
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The C implementation of TypeVar.typing_prepare_subst doesn't catch NameError
when the default is a forward ref — it just raises and returns NULL, so it never
gets a chance to add the default to args. I do both in the except block.

For TypeVarTuple/ParamSpec, their C code delegates to the Python prepare
functions which already handle this themselves, so the outer catch is redundant
for those but harmless.

default = annotationlib.call_evaluate_function(
param.evaluate_default, annotationlib.Format.FORWARDREF)
args = (*args, default)
_check_generic_specialization(cls, args)

new_args = []
Expand Down
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-25-22-30-00.gh-issue-144361.lL9mN2.rst
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Fix :exc:`NameError` when a type parameter default (PEP 696/PEP 749)
refers to a name defined later in the module. Now three call sites in
:mod:`typing` fall back to :func:`annotationlib.call_evaluate_function`
with :data:`~annotationlib.Format.FORWARDREF` when eager evaluation fails.
Loading