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feat: error for Final fields with init=False (#13119) #20382

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feat: error for Final fields with init=False (#13119) #20382

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cfb0993
feat: error for Final fields with init=False (#13119)
Benitex Dec 8, 2025
2c9742b
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Dec 8, 2025
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[pre-commit.ci] auto fixes from pre-commit.com hooks 
for more information, see https://pre-commit.ci
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pre-commit-ci[bot] committed Dec 8, 2025
commit 2c9742b296f0eebb4af6f756313d89372831dfb3
3 changes: 1 addition & 2 deletions mypy/plugins/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -655,8 +655,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
has_post_init = cls.info.get("__post_init__") is not None
if not has_post_init:
self._api.fail(
f'Final field with init=False must have a default value',
stmt.rvalue
"Final field with init=False must have a default value", stmt.rvalue
)

if not has_default and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
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8 changes: 4 additions & 4 deletions test-data/unit/check-dataclasses.test
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Expand Up @@ -2753,7 +2753,7 @@ from dataclasses import dataclass, field

@dataclass
class Foo:
a: Final[int] = field()
a: Final[int] = field()

Foo().a # E: Missing positional argument "a" in call to "Foo"
[builtins fixtures/dataclasses.pyi]
Expand All @@ -2764,9 +2764,9 @@ from dataclasses import dataclass, field

@dataclass
class Foo:
a: Final[int] = field()
a: Final[int] = field()

Foo(1).a
Foo(1).a
[builtins fixtures/dataclasses.pyi]

[case testFinalFieldPostInit]
Expand All @@ -2780,7 +2780,7 @@ class Foo:
def __post_init__(self):
self.a = 1

Foo().a
Foo().a
[builtins fixtures/dataclasses.pyi]

[case testFinalFieldInitFalseWithDefault]
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