Fix recently added enum value type prediction

In python#9443, some code was added to predict the type of enum values where
it is not explicitly when all enum members have the same type.

However, it didn't consider that subclasses of Enum that have a custom
__new__ implementation may use any type for the enum value (typically it
would use only one of their parameters instead of a whole tuple that is
specified in the definition of the member). Fix this by avoiding to
guess the enum value type in classes that implement __new__.

In addition, the added code was buggy in that it didn't only consider
class attributes as enum members, but also instance attributes assigned
to self.* in __init__. Fix this by ignoring implicit nodes when checking
the enum members.

Fixes python#10000.

mypy/plugins/enums.py

@@ -15,6 +15,7 @@
 
 import mypy.plugin  # To avoid circular imports.
 from mypy.types import Type, Instance, LiteralType, CallableType, ProperType, get_proper_type
+from mypy.nodes import TypeInfo
 
 # 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.
@@ -103,6 +104,17 @@ def _infer_value_type_with_auto_fallback(
     return ctx.default_attr_type
 
 
+def _implements_new(info: TypeInfo) -> bool:
+    """Check whether __new__ comes from enum.Enum or was implemented in a
+    subclass. In the latter case, we must infer Any as long as mypy can't infer
+    the type of _value_ from assignments in __new__.
+    """
+    type_with_new = _first(ti for ti in info.mro if ti.names.get('__new__'))
+    if type_with_new is None:
+        return False
+    return type_with_new.fullname != 'enum.Enum'
+
+
 def enum_value_callback(ctx: 'mypy.plugin.AttributeContext') -> Type:
     """This plugin refines the 'value' attribute in enums to refer to
     the original underlying value. For example, suppose we have the
@@ -135,12 +147,22 @@ class SomeEnum:
         # The value-type is still known.
         if isinstance(ctx.type, Instance):
             info = ctx.type.type
+
+            # As long as mypy doesn't understand attribute creation in __new__,
+            # there is no way to predict the value type if the enum class has a
+            # custom implementation
+            if _implements_new(info):
+                return ctx.default_attr_type
+
             stnodes = (info.get(name) for name in info.names)
-            # Enums _can_ have methods.
-            # Omit methods for our value inference.
+
+            # Enums _can_ have methods and instance attributes.
+            # Omit methods and attributes created by assigning to self.*
+            # for our value inference.
             node_types = (
                 get_proper_type(n.type) if n else None
-                for n in stnodes)
+                for n in stnodes
+                if n is None or not n.implicit)
             proper_types = (
                 _infer_value_type_with_auto_fallback(ctx, t)
                 for t in node_types
@@ -158,6 +180,13 @@ class SomeEnum:
 
     assert isinstance(ctx.type, Instance)
     info = ctx.type.type
+
+    # As long as mypy doesn't understand attribute creation in __new__,
+    # there is no way to predict the value type if the enum class has a
+    # custom implementation
+    if _implements_new(info):
+        return ctx.default_attr_type
+
     stnode = info.get(enum_field_name)
     if stnode is None:
         return ctx.default_attr_type

test-data/unit/check-enum.test

@@ -1243,3 +1243,59 @@ class Comparator(enum.Enum):
 
 reveal_type(Comparator.__foo__)  # N: Revealed type is 'builtins.dict[builtins.str, builtins.int]'
 [builtins fixtures/dict.pyi]
+
+[case testEnumWithInstanceAttributes]
+from enum import Enum
+class Foo(Enum):
+    def __init__(self, value: int) -> None:
+        self.foo = "bar"
+    A = 1
+    B = 2
+
+a = Foo.A
+reveal_type(a.value)    # N: Revealed type is 'builtins.int'
+reveal_type(a._value_)  # N: Revealed type is 'builtins.int'
+
+[case testNewSetsUnexpectedValueType]
+from enum import Enum
+
+class bytes:
+    def __new__(cls): pass
+
+class Foo(bytes, Enum):
+    def __new__(cls, value: int) -> 'Foo':
+        obj = bytes.__new__(cls)
+        obj._value_ = "Number %d" % value
+        return obj
+    A = 1
+    B = 2
+
+a = Foo.A
+reveal_type(a.value)    # N: Revealed type is 'Any'
+reveal_type(a._value_)  # N: Revealed type is 'Any'
+[builtins fixtures/__new__.pyi]
+[builtins fixtures/primitives.pyi]
+[typing fixtures/typing-medium.pyi]
+
+[case testValueTypeWithNewInParentClass]
+from enum import Enum
+
+class bytes:
+    def __new__(cls): pass
+
+class Foo(bytes, Enum):
+    def __new__(cls, value: int) -> 'Foo':
+        obj = bytes.__new__(cls)
+        obj._value_ = "Number %d" % value
+        return obj
+
+class Bar(Foo):
+    A = 1
+    B = 2
+
+a = Bar.A
+reveal_type(a.value)    # N: Revealed type is 'Any'
+reveal_type(a._value_)  # N: Revealed type is 'Any'
+[builtins fixtures/__new__.pyi]
+[builtins fixtures/primitives.pyi]
+[typing fixtures/typing-medium.pyi]