Change in semantics and much worse performance for enum members.

[markshannon]

Given the enum:

class Colours(Enum):
    RED = 1

In Python 3.9 and 3.10:

>>> Colours.__dict__["RED"] is Colours.RED
True
>>> Colours.RED.RED
<Colours.RED: 1>

In Python 3.11:

>>> Colours.__dict__["RED"] is Colours.RED
False
>>> Colours.RED.RED
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mark/repos/cpython/Lib/enum.py", line 198, in __get__
    raise AttributeError(
    ^^^^^^^^^^^^^^^^^^^^^
AttributeError: <enum 'Colours'> member has no attribute 'RED'

While these might seem like minor semantic changes, there is also a large performance impact.
Lookup of Colours.RED is simple and efficient in 3.10, but involves a lot of indirection and dispatching through the enum.property class in 3.11.
The performance impact is likely to get worse in 3.12, as we optimize more kinds of attributes.

Introduced in c314e60, I believe.

@pablogsal
@ethanfurman

Linked PRs

• gh-93910: [Enum] remove member.member deprecation #103236
• [3.11] gh-93910: [Enum] remove member.member deprecation (GH-103236) #103299

[ethanfurman]

Those changes are intentional, and remedy an issue present since 3.5. As for performance, the work @markshannon and others have done has already had a considerable impact on making enum faster in 3.12.

[markshannon]

What issue does the change remedy?

I'm puzzled by your claim that the work we've done has sped up enums for 3.12? It hasn't. Without c314e60, it would have.

[markshannon]

It looks like the change in Colours.RED.RED has been brought up before: #87328
Failing on 3.11 seems to be in violation of PEP 387. A warning should be issued until 3.12.

What I don't understand is why this change is necessary at all.
What benefits does it bring? Accessing class attributes from instances is fairly universal, why should enums be special?

[ethanfurman]

The rudimentary timings I did on my system for member access showed enums steadily getting faster over the releases, with a big jump in performance between 3.11 and 3.12.

The Colors.RED.RED behavior was illegal/missing in 3.4, added in 3.5 for performance (but warned against), and SC permission recieved for skipping the programmatic warning in 3.11

Enums are special, and this change brings them back into line with the original, intended, specification.

[markshannon]

The performance of enum lookup is much slower than it should be in 3.11.
It would be a shame to have to advise people not to use enums if they care about performance.

The SC permission pertains to changes in repr(), str(), and format(), AFAICT. No mention is made of the the change in semantics of attribute lookup and performance.

@warsaw Could you confirm?