Issue warning for enum with no members in stub (python#18068)

Follow up to python#17207

CHANGELOG.md

@@ -2,6 +2,46 @@
 
 ## Next release
 
+### Change to enum membership semantics
+
+As per the updated [typing specification for enums](https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members),
+enum members must be left unannotated.
+
+```python
+class Pet(Enum):
+    CAT = 1  # Member attribute
+    DOG = 2  # Member attribute
+    WOLF: int = 3  # New error: Enum members must be left unannotated
+
+    species: str  # Considered a non-member attribute
+```
+
+In particular, the specification change can result in issues in type stubs (`.pyi` files), since
+historically it was common to leave the value absent:
+
+```python
+# In a type stub (.pyi file)
+
+class Pet(Enum):
+    # Change in semantics: previously considered members, now non-member attributes
+    CAT: int
+    DOG: int
+
+    # Mypy will now issue a warning if it detects this situation in type stubs:
+    # > Detected enum "Pet" in a type stub with zero members.
+    # > There is a chance this is due to a recent change in the semantics of enum membership.
+    # > If so, use `member = value` to mark an enum member, instead of `member: type`
+
+class Pet(Enum):
+    # As per the specification, you should now do one of the following:
+    DOG = 1  # Member attribute with value 1 and known type
+    WOLF = cast(int, ...)  # Member attribute with unknown value but known type
+    LION = ...  # Member attribute with unknown value and unknown type
+```
+
+Contributed by Terence Honles in PR [17207](https://github.com/python/mypy/pull/17207) and
+Shantanu Jain in PR [18068](https://github.com/python/mypy/pull/18068).
+
 ## Mypy 1.13
 
 We’ve just uploaded mypy 1.13 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)).

mypy/checker.py

@@ -2588,20 +2588,30 @@ def check_typevar_defaults(self, tvars: Sequence[TypeVarLikeType]) -> None:
 
     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)
+        if defn.info.fullname not in ENUM_BASES and "__members__" in defn.info.names:
+            sym = defn.info.names["__members__"]
+            if isinstance(sym.node, Var) and sym.node.has_explicit_value:
+                # `__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)
 
+        if self.is_stub and self.tree.fullname not in {"enum", "_typeshed"}:
+            if not defn.info.enum_members:
+                self.fail(
+                    f'Detected enum "{defn.info.fullname}" in a type stub with zero members. '
+                    "There is a chance this is due to a recent change in the semantics of "
+                    "enum membership. If so, use `member = value` to mark an enum member, "
+                    "instead of `member: type`",
+                    defn,
+                )
+                self.note(
+                    "See https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members",
+                    defn,
+                )
+
         self.check_enum_bases(defn)
         self.check_enum_new(defn)
 

mypy/checkmember.py

@@ -17,6 +17,7 @@
     ARG_POS,
     ARG_STAR,
     ARG_STAR2,
+    EXCLUDED_ENUM_ATTRIBUTES,
     SYMBOL_FUNCBASE_TYPES,
     Context,
     Decorator,
@@ -48,7 +49,6 @@
     type_object_type_from_function,
 )
 from mypy.types import (
-    ENUM_REMOVED_PROPS,
     AnyType,
     CallableType,
     DeletedType,
@@ -1173,7 +1173,7 @@ def analyze_enum_class_attribute_access(
     itype: Instance, name: str, mx: MemberContext
 ) -> Type | None:
     # Skip these since Enum will remove it
-    if name in ENUM_REMOVED_PROPS:
+    if name in EXCLUDED_ENUM_ATTRIBUTES:
         return report_missing_attribute(mx.original_type, itype, name, mx)
     # Dunders and private names are not Enum members
     if name.startswith("__") and name.replace("_", "") != "":

mypy/nodes.py

@@ -2902,6 +2902,10 @@ def accept(self, visitor: ExpressionVisitor[T]) -> T:
     }
 )
 
+# Attributes that can optionally be defined in the body of a subclass of
+# enum.Enum but are removed from the class __dict__ by EnumMeta.
+EXCLUDED_ENUM_ATTRIBUTES: Final = frozenset({"_ignore_", "_order_", "__order__"})
+
 
 class TypeInfo(SymbolNode):
     """The type structure of a single class.
@@ -3229,6 +3233,19 @@ def protocol_members(self) -> list[str]:
                     members.add(name)
         return sorted(members)
 
+    @property
+    def enum_members(self) -> list[str]:
+        return [
+            name
+            for name, sym in self.names.items()
+            if (
+                isinstance(sym.node, Var)
+                and name not in EXCLUDED_ENUM_ATTRIBUTES
+                and not name.startswith("__")
+                and sym.node.has_explicit_value
+            )
+        ]
+
     def __getitem__(self, name: str) -> SymbolTableNode:
         n = self.get(name)
         if n:

mypy/semanal_enum.py

@@ -10,6 +10,7 @@
 from mypy.nodes import (
     ARG_NAMED,
     ARG_POS,
+    EXCLUDED_ENUM_ATTRIBUTES,
     MDEF,
     AssignmentStmt,
     CallExpr,
@@ -30,7 +31,7 @@
 )
 from mypy.options import Options
 from mypy.semanal_shared import SemanticAnalyzerInterface
-from mypy.types import ENUM_REMOVED_PROPS, LiteralType, get_proper_type
+from mypy.types import LiteralType, get_proper_type
 
 # Note: 'enum.EnumMeta' is deliberately excluded from this list. Classes that directly use
 # enum.EnumMeta do not necessarily automatically have the 'name' and 'value' attributes.
@@ -43,7 +44,7 @@
         "value",
         "_name_",
         "_value_",
-        *ENUM_REMOVED_PROPS,
+        *EXCLUDED_ENUM_ATTRIBUTES,
         # Also attributes from `object`:
         "__module__",
         "__annotations__",

mypy/test/teststubtest.py

@@ -1267,9 +1267,9 @@ def test_enum(self) -> Iterator[Case]:
         yield Case(
             stub="""
             class X(enum.Enum):
-                a: int
-                b: str
-                c: str
+                a = ...
+                b = "asdf"
+                c = "oops"
             """,
             runtime="""
             class X(enum.Enum):
@@ -1282,8 +1282,8 @@ class X(enum.Enum):
         yield Case(
             stub="""
             class Flags1(enum.Flag):
-                a: int
-                b: int
+                a = ...
+                b = 2
             def foo(x: Flags1 = ...) -> None: ...
             """,
             runtime="""
@@ -1297,8 +1297,8 @@ def foo(x=Flags1.a|Flags1.b): pass
         yield Case(
             stub="""
             class Flags2(enum.Flag):
-                a: int
-                b: int
+                a = ...
+                b = 2
             def bar(x: Flags2 | None = None) -> None: ...
             """,
             runtime="""
@@ -1312,8 +1312,8 @@ def bar(x=Flags2.a|Flags2.b): pass
         yield Case(
             stub="""
             class Flags3(enum.Flag):
-                a: int
-                b: int
+                a = ...
+                b = 2
             def baz(x: Flags3 | None = ...) -> None: ...
             """,
             runtime="""
@@ -1346,8 +1346,8 @@ class WeirdEnum(enum.Enum):
         yield Case(
             stub="""
             class Flags4(enum.Flag):
-                a: int
-                b: int
+                a = 1
+                b = 2
             def spam(x: Flags4 | None = None) -> None: ...
             """,
             runtime="""
@@ -1362,7 +1362,7 @@ def spam(x=Flags4(0)): pass
             stub="""
             from typing_extensions import Final, Literal
             class BytesEnum(bytes, enum.Enum):
-                a: bytes
+                a = b'foo'
             FOO: Literal[BytesEnum.a]
             BAR: Final = BytesEnum.a
             BAZ: BytesEnum
@@ -1897,7 +1897,7 @@ def test_good_literal(self) -> Iterator[Case]:
 
             import enum
             class Color(enum.Enum):
-                RED: int
+                RED = ...
 
             NUM: Literal[1]
             CHAR: Literal['a']

mypy/typeops.py

@@ -957,16 +957,20 @@ class Status(Enum):
         items = [
             try_expanding_sum_type_to_union(item, target_fullname) for item in typ.relevant_items()
         ]
-    elif isinstance(typ, Instance) and typ.type.fullname == target_fullname:
-        if typ.type.is_enum:
-            items = [LiteralType(name, typ) for name in typ.get_enum_values()]
-        elif typ.type.fullname == "builtins.bool":
+        return make_simplified_union(items, contract_literals=False)
+
+    if isinstance(typ, Instance) and typ.type.fullname == target_fullname:
+        if typ.type.fullname == "builtins.bool":
             items = [LiteralType(True, typ), LiteralType(False, typ)]
-    else:
-        return typ
+            return make_simplified_union(items, contract_literals=False)
+
+        if typ.type.is_enum:
+            items = [LiteralType(name, typ) for name in typ.type.enum_members]
+            if not items:
+                return typ
+            return make_simplified_union(items, contract_literals=False)
 
-    # if the expanded union would be `Never` leave the type as is
-    return typ if not items else make_simplified_union(items, contract_literals=False)
+    return typ
 
 
 def try_contracting_literals_in_union(types: Sequence[Type]) -> list[ProperType]:
@@ -990,7 +994,7 @@ def try_contracting_literals_in_union(types: Sequence[Type]) -> list[ProperType]
                 if fullname not in sum_types:
                     sum_types[fullname] = (
                         (
-                            set(typ.fallback.get_enum_values())
+                            set(typ.fallback.type.enum_members)
                             if typ.fallback.type.is_enum
                             else {True, False}
                         ),
@@ -1023,7 +1027,7 @@ def coerce_to_literal(typ: Type) -> Type:
         if typ.last_known_value:
             return typ.last_known_value
         elif typ.type.is_enum:
-            enum_values = typ.get_enum_values()
+            enum_values = typ.type.enum_members
             if len(enum_values) == 1:
                 return LiteralType(value=enum_values[0], fallback=typ)
     return original_type

mypy/types.py

@@ -150,10 +150,6 @@
 
 OVERLOAD_NAMES: Final = ("typing.overload", "typing_extensions.overload")
 
-# Attributes that can optionally be defined in the body of a subclass of
-# enum.Enum but are removed from the class __dict__ by EnumMeta.
-ENUM_REMOVED_PROPS: Final = ("_ignore_", "_order_", "__order__")
-
 NEVER_NAMES: Final = (
     "typing.NoReturn",
     "typing_extensions.NoReturn",
@@ -1559,23 +1555,10 @@ def is_singleton_type(self) -> bool:
         # Also make this return True if the type corresponds to NotImplemented?
         return (
             self.type.is_enum
-            and len(self.get_enum_values()) == 1
+            and len(self.type.enum_members) == 1
             or self.type.fullname in {"builtins.ellipsis", "types.EllipsisType"}
         )
 
-    def get_enum_values(self) -> list[str]:
-        """Return the list of values for an Enum."""
-        return [
-            name
-            for name, sym in self.type.names.items()
-            if (
-                isinstance(sym.node, mypy.nodes.Var)
-                and name not in ENUM_REMOVED_PROPS
-                and not name.startswith("__")
-                and sym.node.has_explicit_value
-            )
-        ]
-
 
 class FunctionLike(ProperType):
     """Abstract base class for function types."""

mypyc/irbuild/classdef.py

@@ -7,6 +7,7 @@
 from typing import Callable, Final
 
 from mypy.nodes import (
+    EXCLUDED_ENUM_ATTRIBUTES,
     TYPE_VAR_TUPLE_KIND,
     AssignmentStmt,
     CallExpr,
@@ -27,7 +28,7 @@
     TypeParam,
     is_class_var,
 )
-from mypy.types import ENUM_REMOVED_PROPS, Instance, UnboundType, get_proper_type
+from mypy.types import Instance, UnboundType, get_proper_type
 from mypyc.common import PROPSET_PREFIX
 from mypyc.ir.class_ir import ClassIR, NonExtClassInfo
 from mypyc.ir.func_ir import FuncDecl, FuncSignature
@@ -683,7 +684,7 @@ def add_non_ext_class_attr(
             cdef.info.bases
             and cdef.info.bases[0].type.fullname == "enum.Enum"
             # Skip these since Enum will remove it
-            and lvalue.name not in ENUM_REMOVED_PROPS
+            and lvalue.name not in EXCLUDED_ENUM_ATTRIBUTES
         ):
             # Enum values are always boxed, so use object_rprimitive.
             attr_to_cache.append((lvalue, object_rprimitive))

test-data/unit/check-enum.test

@@ -1788,14 +1788,17 @@ import lib
 
 [file lib.pyi]
 from enum import Enum
-class A(Enum):
+class A(Enum):  # E: Detected enum "lib.A" in a type stub with zero members. There is a chance this is due to a recent change in the semantics of enum membership. If so, use `member = value` to mark an enum member, instead of `member: type` \
+                # N: See https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members
     x: int
 class B(A):  # E: Cannot extend enum with existing members: "A"
     x = 1    # E: Cannot override writable attribute "x" with a final one
 
 class C(Enum):
     x = 1
-class D(C):  # E: Cannot extend enum with existing members: "C"
+class D(C):  # E: Cannot extend enum with existing members: "C" \
+             # E: Detected enum "lib.D" in a type stub with zero members. There is a chance this is due to a recent change in the semantics of enum membership. If so, use `member = value` to mark an enum member, instead of `member: type` \
+             # N: See https://typing.readthedocs.io/en/latest/spec/enums.html#defining-members
     x: int   # E: Cannot assign to final name "x"
 [builtins fixtures/bool.pyi]