Enable Final instance attributes for attrs

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
@@ -756,20 +756,28 @@ 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 = []
+ sym_table = ctx.cls.info.names
  for attribute in attributes:
  if not attribute.init:
  continue
  if attribute.kw_only:
  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 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

test-data/unit/check-attr.test

@@ -1834,3 +1834,36 @@ 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]
+
+[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]