From 1f09e3e25cb2a0772cd47e8b512f5f4f0df194b5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tin=20Tvrtkovi=C4=87?= <tinchester@gmail.com>
Date: Fri, 2 Dec 2022 00:14:55 +0100
Subject: [PATCH 1/2] Enable Final instance attributes for attrs

---
 mypy/plugins/attrs.py          | 16 +++++++++++++---
 test-data/unit/check-attr.test | 14 ++++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/mypy/plugins/attrs.py b/mypy/plugins/attrs.py
index ce0f45967152c..67d8001d8421a 100644
--- a/mypy/plugins/attrs.py
+++ b/mypy/plugins/attrs.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from typing import Iterable, List, cast
-from typing_extensions import Final
+from typing_extensions import Final, Literal
 
 import mypy.plugin  # To avoid circular imports.
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
@@ -756,10 +756,10 @@ def _add_init(
     ctx: mypy.plugin.ClassDefContext,
     attributes: list[Attribute],
     adder: MethodAdder,
-    method_name: str,
+    method_name: Literal["__init__", "__attrs_init__"],
 ) -> None:
     """Generate an __init__ method for the attributes and add it to the class."""
-    # Convert attributes to arguments with kw_only arguments at the  end of
+    # Convert attributes to arguments with kw_only arguments at the end of
     # the argument list
     pos_args = []
     kw_only_args = []
@@ -770,6 +770,16 @@ def _add_init(
             kw_only_args.append(attribute.argument(ctx))
         else:
             pos_args.append(attribute.argument(ctx))
+
+        # If the attribute is Final, present in `__init__` and has
+        # no default, make sure it doesn't error later.
+        if not attribute.has_default:
+            for ti in ctx.cls.info.mro:
+                if attribute.name in ti.names:
+                    sym_node = ti.names[attribute.name].node
+                    if isinstance(sym_node, Var) and sym_node.is_final:
+                        sym_node.final_set_in_init = True
+                        break
     args = pos_args + kw_only_args
     if all(
         # We use getattr rather than instance checks because the variable.type
diff --git a/test-data/unit/check-attr.test b/test-data/unit/check-attr.test
index 4d27d5f39d1e7..096b6815b55bd 100644
--- a/test-data/unit/check-attr.test
+++ b/test-data/unit/check-attr.test
@@ -1834,3 +1834,17 @@ class Sub(Base):
 # This matches runtime semantics
 reveal_type(Sub)  # N: Revealed type is "def (*, name: builtins.str, first_name: builtins.str, last_name: builtins.str) -> __main__.Sub"
 [builtins fixtures/property.pyi]
+
+[case testFinalInstanceAttribute]
+from attrs import define
+from typing import Final
+
+@define
+class C:
+    a: Final[int]
+
+reveal_type(C) # N: Revealed type is "def (a: builtins.int) -> __main__.C"
+
+C(1).a = 2 # E: Cannot assign to final attribute "a"
+
+[builtins fixtures/property.pyi]

From 9acbca0dfc6ba68ffdd6ffb1745f35a62c45fbb5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tin=20Tvrtkovi=C4=87?= <tinchester@gmail.com>
Date: Tue, 6 Dec 2022 00:56:32 +0100
Subject: [PATCH 2/2] Don't search the MRO

---
 mypy/plugins/attrs.py          | 12 +++++-------
 test-data/unit/check-attr.test | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/mypy/plugins/attrs.py b/mypy/plugins/attrs.py
index 67d8001d8421a..16e8891e5f575 100644
--- a/mypy/plugins/attrs.py
+++ b/mypy/plugins/attrs.py
@@ -763,6 +763,7 @@ def _add_init(
     # the argument list
     pos_args = []
     kw_only_args = []
+    sym_table = ctx.cls.info.names
     for attribute in attributes:
         if not attribute.init:
             continue
@@ -773,13 +774,10 @@ def _add_init(
 
         # If the attribute is Final, present in `__init__` and has
         # no default, make sure it doesn't error later.
-        if not attribute.has_default:
-            for ti in ctx.cls.info.mro:
-                if attribute.name in ti.names:
-                    sym_node = ti.names[attribute.name].node
-                    if isinstance(sym_node, Var) and sym_node.is_final:
-                        sym_node.final_set_in_init = True
-                        break
+        if not attribute.has_default and attribute.name in sym_table:
+            sym_node = sym_table[attribute.name].node
+            if isinstance(sym_node, Var) and sym_node.is_final:
+                sym_node.final_set_in_init = True
     args = pos_args + kw_only_args
     if all(
         # We use getattr rather than instance checks because the variable.type
diff --git a/test-data/unit/check-attr.test b/test-data/unit/check-attr.test
index 096b6815b55bd..f555f2ea70111 100644
--- a/test-data/unit/check-attr.test
+++ b/test-data/unit/check-attr.test
@@ -1848,3 +1848,22 @@ reveal_type(C) # N: Revealed type is "def (a: builtins.int) -> __main__.C"
 C(1).a = 2 # E: Cannot assign to final attribute "a"
 
 [builtins fixtures/property.pyi]
+
+[case testFinalInstanceAttributeInheritance]
+from attrs import define
+from typing import Final
+
+@define
+class C:
+    a: Final[int]
+
+@define
+class D(C):
+    b: Final[str]
+
+reveal_type(D) # N: Revealed type is "def (a: builtins.int, b: builtins.str) -> __main__.D"
+
+D(1, "").a = 2 # E: Cannot assign to final attribute "a"
+D(1, "").b = "2" # E: Cannot assign to final attribute "b"
+
+[builtins fixtures/property.pyi]
\ No newline at end of file