(🐞) Enum.name is Any in 3.11 / support enum.property

[KotlinIsland]

from enum import Enum

value: Enum

reveal_type(value)
reveal_type(value.name)

> mypy --python-version 3.10 test.py
test.py:5: note: Revealed type is "enum.Enum"
test.py:6: note: Revealed type is "str"
Success: no issues found in 1 source file

> mypy --python-version 3.11 test.py
test.py:5: note: Revealed type is "enum.Enum"
test.py:6: note: Revealed type is "Any"
Success: no issues found in 1 source file

in 3.11 name is decorated with enum.property which is a subtype of types.DynamicClassAttribute (aka, builtins.property)

from python/typeshed#7564
Related #12220

[jhance]

To paraphase in a way others won't need to dig throuhg typeshed, typeshed does this:

if sys.version_info >= (3, 11):
    class property(types.DynamicClassAttribute):
        ...
    _magic_enum_attr = property
else:
    _magic_enum_attr = types.DynamicClassAttribute

[JelleZijlstra]

And types.DynamicClassAttribute is an alias for builtins.property.

[AlexWaygood]

It might be worth pointing out that mypy doesn't get the behaviour precisely correct on Python <=3.10 either:

from enum import Enum

class Foo(Enum):
    BAR = 'bar'
    BAZ = 'baz'

reveal_type(Foo.name)  # No error from mypy, raises AttributeError at runtime
reveal_type(Foo.value)  # No error from mypy, raises AttributeError at runtime

On Python <= 3.10, Enum.name and Enum.value are special-cased by the enum module to allow this magical behaviour, where name and value are read-only properties that raise AttributeError if accessed from the class rather than an instance. This is to allow enumerations where members have name or value as their names:

>>> from enum import Enum
>>> class Dictionary(Enum):
...     name = 'a word used to identify something'
...     animal = 'a living creature'
...
>>> Dictionary.name
<Dictionary.name: 'a word used to identify something'>
>>> Dictionary.name.name
'name'

In Python 3.11, name and value are no longer special-cased by the enum module. Instead, the new enum.property class generalises this behaviour, so that users can create their own "magic enum properties":

>>> import enum
>>> class CustomEnum(enum.Enum):
...     @enum.property
...     def foo(self):
...         return 'MAGIC'
...
>>> class MyEnum(CustomEnum):
...     foo = 'foo'
...
>>> MyEnum.foo
<MyEnum.foo: 'foo'>
>>> MyEnum.foo.foo
'MAGIC'

[AlexWaygood]

Here's the dedicated issue regarding the incorrect behaviour on <=3.10: #4654

[playpauseandstop]

Hope this is proper place to report, as I have experienced another issue related to Enum.name behaviour after upgrading my codebase to Python 3.11 (and latest mypy 0.991).

With Python 3.10 following code checked OK,

from enum import Enum, unique


@unique
class SortEnum(Enum):
     name = "name"
     name_desc = "-name"

❯ pipx run mypy==0.991 --python-version=3.10 test.py
Success: no issues found in 1 source file

But with Python 3.11 it throwing,

❯ pipx run mypy==0.991 --python-version=3.11 test.py
test.py:6:12: error: Incompatible types in assignment (expression has type "str", base class "Enum" defined the type as "Callable[[Enum], str]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

As a quick workaround it might be possible to rename SortEnum.name to SortEnum.name_asc, and in meantime looking forward to proper fix 🙏

[gimbo]

name is still problematic in that it can't be used with an auto() value:

from enum import Enum, auto


class E(Enum):
    name = auto()

$ mypy /tmp/foo.py
/tmp/foo.py:5: error: Incompatible types in assignment (expression has type "auto", base class "Enum" defined the type as "str")  [assignment]
Found 1 error in 1 file (checked 1 source file)

(Not a problem for me btw — I just happened to notice.)

$ python --version
Python 3.11.1

$ mypy --version
mypy 1.0.0 (compiled: yes)