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

Test cases from PR #14285, thanks to @jakezych.

Co-Authored-By: Jake Zych <[email protected]>
Co-Authored-By: Rafael Christof <[email protected]>

Author: 3 people

mypy/plugins/dataclasses.py

@@ -651,6 +651,14 @@ def collect_attributes(self) -> list[DataclassAttribute] | None:
             elif not isinstance(stmt.rvalue, TempNode):
                 has_default = True
 
+            if node.is_final and not is_in_init and not has_default:
+                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
+                    )
+
             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

test-data/unit/check-dataclasses.test

@@ -2737,3 +2737,59 @@ class ClassB(ClassA):
     def value(self) -> int:
         return 0
 [builtins fixtures/dict.pyi]
+
+[case testErrorFinalFieldNoInitNoArgumentPassed]
+from typing import Final
+from dataclasses import dataclass, field
+@dataclass
+class Foo:
+    a: Final[int] = field(init=False) # E: Final field with init=False must have a default value
+Foo().a
+[builtins fixtures/dataclasses.pyi]
+
+[case testErrorFinalFieldInitNoArgumentPassed]
+from typing import Final
+from dataclasses import dataclass, field
+
+@dataclass
+class Foo:
+    a: Final[int] = field() 
+
+Foo().a # E: Missing positional argument "a" in call to "Foo"
+[builtins fixtures/dataclasses.pyi]
+
+[case testFinalFieldGeneratedInitArgumentPassed]
+from typing import Final
+from dataclasses import dataclass, field
+
+@dataclass
+class Foo:
+    a: Final[int] = field() 
+
+Foo(1).a 
+[builtins fixtures/dataclasses.pyi]
+
+[case testFinalFieldPostInit]
+from typing import Final
+from dataclasses import dataclass, field
+
+@dataclass
+class Foo:
+    a: Final[int] = field(init=False)
+
+    def __post_init__(self):
+        self.a = 1
+
+Foo().a 
+[builtins fixtures/dataclasses.pyi]
+
+[case testFinalFieldInitFalseWithDefault]
+from typing import Final
+from dataclasses import dataclass, field
+
+@dataclass
+class Foo:
+    a: Final[int] = field(init=False, default=1)
+
+Foo().a
+[builtins fixtures/dataclasses.pyi]