From df2a1e123ef17536825f7e94092929b9aeeb0c04 Mon Sep 17 00:00:00 2001
From: Dipanshu Ambilkar <dipanshuambilker@gmail.com>
Date: Thu, 29 Oct 2026 00:01:13 +0530
Subject: [PATCH 1/2] Report final dataclass fields that are never initialized
 (#13119)

A `Final` dataclass attribute can only be assigned inside `__init__()`. If
the field is declared with `field(init=False)` and has neither a `default`
nor a `default_factory`, nothing ever sets it, so reading the attribute
fails with `AttributeError` at runtime.

The `field(...)` call counted as an initializer, so the "Final name must be
initialized with a value" check was skipped. Report the field instead when it
is final, excluded from the generated `__init__()` and has no default value.
---
 mypy/plugins/dataclasses.py           | 15 +++++++++++++++
 test-data/unit/check-dataclasses.test | 21 +++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index 7a6a3876d9bfd..a97a7fdfafba0 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -654,6 +654,21 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             elif not isinstance(stmt.rvalue, TempNode):
                 has_default = True
 
+            if (
+                node.is_final
+                and has_field_call
+                and not is_in_init
+                and not has_default
+                and not is_init_var
+                and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES
+            ):
+                # A final attribute can only be assigned in __init__(), so if it is
+                # excluded from __init__() and has no default value, it can never be set.
+                self._api.fail(
+                    "Final field must have a default value if init=False",
+                    stmt.rvalue,
+                )
+
             if not has_default and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
                 # Make all non-default dataclass attributes implicit because they are de-facto
                 # set on self in the generated __init__(), not in the class body. On the other
diff --git a/test-data/unit/check-dataclasses.test b/test-data/unit/check-dataclasses.test
index d269ad2bad0ec..0549f1d3e0a04 100644
--- a/test-data/unit/check-dataclasses.test
+++ b/test-data/unit/check-dataclasses.test
@@ -2952,6 +2952,27 @@ class Bad2:
     x: InitVar[Final[int]]  # E: Final can be only used as an outermost qualifier in a variable annotation
 [builtins fixtures/dict.pyi]
 
+[case testDataclassFinalFieldNotInInit]
+from dataclasses import dataclass, field
+from typing import Final
+
+@dataclass
+class Bad:
+    x: Final[int] = field(init=False)  # E: Final field must have a default value if init=False
+
+@dataclass
+class GoodDefault:
+    x: Final[int] = field(init=False, default=0)
+
+@dataclass
+class GoodFactory:
+    x: Final[int] = field(init=False, default_factory=int)
+
+@dataclass
+class GoodPlain:
+    x: Final[int] = 0
+[builtins fixtures/tuple.pyi]
+
 [case testNoCrashOnNestedSpecialFormsWithAnnotations]
 import dataclasses
 from typing import ClassVar, NamedTuple

From b885572e9aad2accd8ba6eeb4578d3ed793d8a0e Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 28 Oct 2026 18:33:18 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/plugins/dataclasses.py | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index a97a7fdfafba0..1325f193bd1f2 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -664,10 +664,7 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             ):
                 # A final attribute can only be assigned in __init__(), so if it is
                 # excluded from __init__() and has no default value, it can never be set.
-                self._api.fail(
-                    "Final field must have a default value if init=False",
-                    stmt.rvalue,
-                )
+                self._api.fail("Final field must have a default value if init=False", stmt.rvalue)
 
             if not has_default and self._spec is _TRANSFORM_SPEC_FOR_DATACLASSES:
                 # Make all non-default dataclass attributes implicit because they are de-facto