[3.12] gh-109022: [Enum] require names=() to create empty enum type (GH-109048)

[miss-islington]

add guard so that Enum('bar') raises a TypeError instead of
creating a new enum class called bar. To create the new but
empty class, use:

huh = Enum('bar', names=())

(cherry picked from commit c74e440)

Co-authored-by: Ethan Furman ethan@stoneleaf.us

• Issue: Instantiation of an empty Enum with any value no longer throws ValueError #109022

[Yhg1s]

I don't consider this a suitable change at this stage. (In fact, I wouldn't consider this a suitable change after beta 1). How is this not an API break? How can this be backward-compatible enough for a patch release?

If it really is backward-compatible enough to include in a patch release (which requires some convincing), does it need to go into 3.12.0, or can it be delayed to 3.12.1? If it needs to go into 3.12.0, we'll need a new release candidate.

[ethanfurman]

Short answer: it's restoring 3.11 behavior.

Python 3.11.4+ (heads/3.11:0aa3b9d76c, Aug  8 2023, 10:35:56) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from enum import Enum
>>> huh = Enum('bar')
Traceback (most recent call last):
  ...
ValueError: 'bar' is not a valid Enum
>>> huh = Enum('bar', names=())
>>> huh
<enum 'bar'>

---

Long answer:

When something like MyEnum(12) is executed, EnumType.__call__(cls, value, names, ...) is called and has to determine whether this is a value lookup on an already defined enum, or an attempt to create a new enum type.

In 3.11- this decision was based on whether names was populated (empty names --> value lookup). However, this led to confusing error messages when a value was a tuple:

from enum import Enum

class MyTupleEnum(Enum):
    first = 1, 10

>>> MyTupleEnum(1, 10)
Traceback (most recent call last):
  ...
TypeError: <enum 1> cannot extend <enum 'MyTupleEnum'>

Since enums with members cannot be further subclassed, a better check in EnumType.__call__ is whether the enum already has members, but that led to the current bug:

Python 3.12.0rc2+ (heads/3.12:fff524ede8, Sep  8 2023, 06:47:36) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from enum import Enum
>>> Enum('bar')
<enum 'bar'>

The bug here is that it looks like a value lookup (and has been a value lookup since enum's inception), and it used to be an error, but now it is actually creating a new enum type.

In 3.11-, creating a new enum using the functional syntax required supplying names; in 3.12, the edge case is creating a new enum, but without members and without any new behavior, will work without supplying names -- and that's the bug.

---

Having said all that, it is an edge case, and can probably wait until 3.12.1.

[ethanfurman]

@Yhg1s, The reason this is not an API break is two-fold:

• the error crept in with the last bug-fix in rc1;
• names is still a positional-or-keyword argument -- I just show it in the keyword form because it's easier to read.

[Yhg1s]

It's not exactly restoring 3.11 behaviour, since it raises a ValueError in 3.11, but I see it raised TypeError in earlier 3.12 pre-releases as well.

[ethanfurman]

@Yhg1s, thanks for merging! And thanks for all your work (even if you hadn't merged) -- I appreciate your thoroughness.