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Enum regression: AttributeError when accessing class variables on instances #87328Description
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GitHub fields:
assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2021-03-03.19:42:57.050>
created_at = <Date 2021-02-08.12:22:59.718>
labels = ['type-bug', 'library', '3.10']
title = 'Enum regression: AttributeError when accessing class variables on instances'
updated_at = <Date 2021-03-03.22:22:26.284>
user = 'https://github.com/hroncok' bugs.python.org fields:
activity = <Date 2021-03-03.22:22:26.284>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2021-03-03.19:42:57.050>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2021-02-08.12:22:59.718>
creator = 'hroncok'
dependencies = []
files = []
hgrepos = []
issue_num = 43162
keywords = ['patch']
message_count = 13.0
messages = ['386626', '386628', '386642', '386668', '386672', '386674', '386680', '386683', '386700', '387953', '388038', '388053', '388063']
nosy_count = 3.0
nosy_names = ['ethan.furman', 'hroncok', 'baker.dylan.c']
pr_nums = ['24486', '24487']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43162'
versions = ['Python 3.10'] Activityhroncok commented on Feb 8, 2021 MannequinAuthor More actions I believe I found a regression in Enum in Python 3.10.0a5.
This is Python 3.9:
>>> import enum
>>> class C(enum.Enum):
... A = 0
... B = 1
...
>>> C.A
<C.A: 0>
>>> C.B
<C.B: 1>
>>> C(0).A
<C.A: 0>
>>> C(0).B
<C.B: 1>
>>> The Enum instances can access class-attributes via dot, like normal instances do.
While in Python 3.10.0a5:
>>> import enum
>>> class C(enum.Enum):
... A = 0
... B = 1
...
>>> C.A
<C.A: 0>
>>> C.B
<C.B: 1>
>>> C(0).A
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib64/python3.10/enum.py", line 146, in __get__
raise AttributeError(
AttributeError: C: no attribute 'A'
>>> C(0).B
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib64/python3.10/enum.py", line 146, in __get__
raise AttributeError(
AttributeError: C: no attribute 'B'In real word situations, it breaks meson:
https://github.com/mesonbuild/meson/blob/398df5629863e913fa603cbf02c525a9f501f8a8/mesonbuild/backend/backends.py#L52-L78
The __str__ method does:
if self is self.EXITCODE: ... And it fails with:
AttributeError: TestProtocol: no attribute 'EXITCODE'
This worked with 3.10.0a4.
If this is a deliberate backwards incompatible change of behavior, I don't think it is documented in the changelog or what's new in Python 3.10, nor that it was deprecated in Python 3.9 and 3.8. hroncok commented on Feb 8, 2021 MannequinAuthor More actions Git bisect:
c314e60 is the first new commit
commit c314e60
Author: Ethan Furman <[email protected]>
Date: Tue Jan 12 23:47:57 2021 -0800
bpo-42901: [Enum] move member creation to `__set_name__` (GH-24196)
`type.__new__` calls `__set_name__` and `__init_subclass__`, which means
that any work metaclasses do after calling `super().__new__()` will not
be available to those two methods. In particular, `Enum` classes that
want to make use of `__init_subclass__` will not see any members.
Almost all customization is therefore moved to before the
`type.__new__()` call, including changing all members to a proto member
descriptor with a `__set_name__` that will do the final conversion of a
member to be an instance of the `Enum` class.
Lib/enum.py | 297 +++++++++++++--------
Lib/inspect.py | 2 +-
Lib/test/test_enum.py | 15 +-
.../2021-01-11-17-36-59.bpo-42901.gFd-ta.rst | 3 +
4 files changed, 207 insertions(+), 110 deletions(-)
create mode 100644 Misc/NEWS.d/next/Library/2021-01-11-17-36-59.bpo-42901.gFd-ta.rst ethanfurman commented on Feb 8, 2021 The code for that __str__ seems very inefficient -- why doesn't it just do:
?
-----
The issue is not being able to access class attributes, the issue is whether one enum member should be seen as an attribute of another:
EnumClass.RED.BLUE
and the answer is no. That wasn't possible when Enum was first introduced in 3.4, and was an unfortunate side-effect of speeding up member access in 3.5 (or 3.6). The docs have always warned against it.
A deprecation warning is an easier transition, though, so I'll do that for 3.10, and in 3.11 it will become an error.
Thank you for reporting! :-) hroncok commented on Feb 8, 2021 MannequinAuthor More actions bakerdylanc commented on Feb 9, 2021 Author of said meson code here. I use this pattern when the enum names and values are implementation details, but the string values are user visible. In this case the enum itself is used internally to represent what kind of test we're doing, but we're initializing it from user input. There might be reasons in the future that the names of the enum members and the string values being passed in aren't the same anymore, say because we map two string values to one enum value. I guess I could accomplish the same thing with a staticmethod or helper function, but the code is effectively an alternate initializer, and follows that pattern using a classmethod. hroncok commented on Feb 9, 2021 MannequinAuthor More actions Ethan, should the depreciation exist for 2 releases prior to removal?
Dylan, even in that case, I guess the proper way to access the other members is supposed to be type(self).FOO or ClassName.FOO, not self.FOO. ethanfurman commented on Feb 9, 2021 ethanfurman commented on Feb 9, 2021 Dylan, it's not the from_str() method, but the __str__ method that is the problem. Instead of
def __str__(self):
if self is self.EXITCODE:
return 'exitcode' it should be
def __str__(self):
cls = self.__class__
if self is cls.EXITCODE:
return 'exitcode'hroncok commented on Feb 9, 2021 MannequinAuthor More actions ethanfurman commented on Mar 2, 2021 DeprecationWarning will be active in 3.10 and 3.11 with removal in 3.12. ethanfurman commented on Mar 3, 2021 hroncok commented on Mar 3, 2021 MannequinAuthor More actions ethanfurman commented on Mar 3, 2021 You're welcome. Thank you for pushing the issue! :-)
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