Repository: python/mypy

Issue:
Overriding non-enumeration attributes in Enum subclass fails
**Bug Report**

Inheriting an `Enum`-class and overriding, for example, a dunder-attribute fails to typecheck. Although it works fine during runtime.

**To Reproduce**

```python
from enum import Enum


class BaseEnum(Enum):
    __something__ = 3


class SomeEnum(BaseEnum):
    __something__ = 2
```

**Expected Behavior**

It should typecheck, as it passes during runtime.

**Actual Behavior**

```
error: Cannot override final attribute "__something__" (previously declared in base class "BaseEnum")
```

**Your Environment**

- Mypy version used: `master` (26fdd456b7e665c3c0f161234ee48b9e6144940d)
- Mypy command-line flags: -
- Mypy configuration options from `mypy.ini` (and other config files): 
```
strict_optional = True
```
- Python version used: `3.9.4`
- Operating system and version: MacOS Catalina `10.15.6`

---

The issue has also been briefly mentioned in these places:

- https://github.com/python/mypy/issues/11850#issuecomment-1030679580
- https://github.com/python/mypy/issues/10857#issuecomment-1010459198


Discussion:
@flaeppe are you interested in working on this? 🙂 
I could take a swing at it if I find some time, though I'm not too familiar with mypy code so I'm not sure what'll come out of it. 

If someone else feels like working on it, please go ahead

Resolving patch:
diff --git a/mypy/checker.py b/mypy/checker.py
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -1834,11 +1834,7 @@ def visit_class_def(self, defn: ClassDef) -> None:
         if typ.is_protocol and typ.defn.type_vars:
             self.check_protocol_variance(defn)
         if not defn.has_incompatible_baseclass and defn.info.is_enum:
-            for base in defn.info.mro[1:-1]:  # we don't need self and `object`
-                if base.is_enum and base.fullname not in ENUM_BASES:
-                    self.check_final_enum(defn, base)
-            self.check_enum_bases(defn)
-            self.check_enum_new(defn)
+            self.check_enum(defn)
 
     def check_final_deletable(self, typ: TypeInfo) -> None:
         # These checks are only for mypyc. Only perform some checks that are easier
@@ -1896,6 +1892,24 @@ def check_init_subclass(self, defn: ClassDef) -> None:
             # all other bases have already been checked.
             break
 
+    def check_enum(self, defn: ClassDef) -> None:
+        assert defn.info.is_enum
+        if defn.info.fullname not in ENUM_BASES:
+            for sym in defn.info.names.values():
+                if (isinstance(sym.node, Var) and sym.node.has_explicit_value and
+                        sym.node.name == '__members__'):
+                    # `__members__` will always be overwritten by `Enum` and is considered
+                    # read-only so we disallow assigning a value to it
+                    self.fail(
+                        message_registry.ENUM_MEMBERS_ATTR_WILL_BE_OVERRIDEN, sym.node
+                    )
+        for base in defn.info.mro[1:-1]:  # we don't need self and `object`
+            if base.is_enum and base.fullname not in ENUM_BASES:
+                self.check_final_enum(defn, base)
+
+        self.check_enum_bases(defn)
+        self.check_enum_new(defn)
+
     def check_final_enum(self, defn: ClassDef, base: TypeInfo) -> None:
         for sym in base.names.values():
             if self.is_final_enum_value(sym):
diff --git a/mypy/message_registry.py b/mypy/message_registry.py
--- a/mypy/message_registry.py
+++ b/mypy/message_registry.py
@@ -205,6 +205,11 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
 )
 CANNOT_MAKE_DELETABLE_FINAL: Final = ErrorMessage("Deletable attribute cannot be final")
 
+# Enum
+ENUM_MEMBERS_ATTR_WILL_BE_OVERRIDEN: Final = ErrorMessage(
+    'Assigned "__members__" will be overriden by "Enum" internally'
+)
+
 # ClassVar
 CANNOT_OVERRIDE_INSTANCE_VAR: Final = ErrorMessage(
     'Cannot override instance variable (previously declared on base class "{}") with class '
diff --git a/mypy/semanal.py b/mypy/semanal.py
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -116,6 +116,7 @@
 )
 from mypy.util import (
     correct_relative_import, unmangle, module_prefix, is_typeshed_file, unnamed_function,
+    is_dunder,
 )
 from mypy.scope import Scope
 from mypy.semanal_shared import (
@@ -2480,8 +2481,9 @@ def store_final_status(self, s: AssignmentStmt) -> None:
                     cur_node = self.type.names.get(lval.name, None)
                     if (cur_node and isinstance(cur_node.node, Var) and
                             not (isinstance(s.rvalue, TempNode) and s.rvalue.no_rhs)):
-                        cur_node.node.is_final = True
-                        s.is_final_def = True
+                        # Double underscored members are writable on an `Enum`.
+                        # (Except read-only `__members__` but that is handled in type checker)
+                        cur_node.node.is_final = s.is_final_def = not is_dunder(cur_node.node.name)
 
                 # Special case: deferred initialization of a final attribute in __init__.
                 # In this case we just pretend this is a valid final definition to suppress


Test patch:
diff --git a/test-data/unit/check-enum.test b/test-data/unit/check-enum.test
--- a/test-data/unit/check-enum.test
+++ b/test-data/unit/check-enum.test
@@ -2029,3 +2029,54 @@ class C(Enum):
 
 C._ignore_ # E: "Type[C]" has no attribute "_ignore_"
 [typing fixtures/typing-medium.pyi]
+
+[case testCanOverrideDunderAttributes]
+import typing
+from enum import Enum, Flag
+
+class BaseEnum(Enum):
+    __dunder__ = 1
+    __labels__: typing.Dict[int, str]
+
+class Override(BaseEnum):
+    __dunder__ = 2
+    __labels__ = {1: "1"}
+
+Override.__dunder__ = 3
+BaseEnum.__dunder__ = 3
+Override.__labels__ = {2: "2"}
+
+class FlagBase(Flag):
+    __dunder__ = 1
+    __labels__: typing.Dict[int, str]
+
+class FlagOverride(FlagBase):
+    __dunder__ = 2
+    __labels = {1: "1"}
+
+FlagOverride.__dunder__ = 3
+FlagBase.__dunder__ = 3
+FlagOverride.__labels__ = {2: "2"}
+[builtins fixtures/dict.pyi]
+
+[case testCanNotInitialize__members__]
+import typing
+from enum import Enum
+
+class WritingMembers(Enum):
+    __members__: typing.Dict[Enum, Enum] = {}  # E: Assigned "__members__" will be overriden by "Enum" internally
+
+class OnlyAnnotatedMembers(Enum):
+    __members__: typing.Dict[Enum, Enum]
+[builtins fixtures/dict.pyi]
+
+[case testCanOverrideDunderOnNonFirstBaseEnum]
+import typing
+from enum import Enum
+
+class Some:
+    __labels__: typing.Dict[int, str]
+
+class A(Some, Enum):
+    __labels__ = {1: "1"}
+[builtins fixtures/dict.pyi]