[DipanshuAmbilkar03]

Fixes #13119

Summary

A Final dataclass field can only be assigned inside __init__(). If the field is declared with field(init=False) and has neither a default nor a default_factory, the generated __init__() never sets it, so the attribute is missing at runtime:

from dataclasses import dataclass, field
from typing import Final

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

Foo().a  # AttributeError: 'Foo' object has no attribute 'a'

mypy accepts this today, because the field(...) call counts as an initializer and that is enough to skip the regular "Final name must be initialized with a value" check.

Fix

While collecting dataclass attributes, I report a field when it is final, excluded from the generated __init__() and has neither a default nor a default_factory — that combination means nothing can ever assign the attribute.

Fields with a default, plain Final fields that the generated __init__() sets, and fields of custom dataclass_transform specs keep the current behaviour.

Test

testDataclassFinalFieldNotInInit in test-data/unit/check-dataclasses.test covers the reported case plus the three shapes that must stay accepted: field(init=False, default=0), field(init=False, default_factory=int) and a plain Final[int] = 0.

Verification

• pytest mypy/test/testcheck.py -k "Dataclass or dataclass" — 212 passed.
• pytest mypy/test/testfinegrained.py -k dataclass — 14 passed.
• python -m mypy --config-file mypy_self_check.ini -p mypy — clean.
• Full mypy/test suite is running in CI as part of this PR.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅