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Enum regression: AttributeError when accessing class variables on instances #87328
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hroncok commented
on Feb 8, 2021 hroncokmannequinMannequinAuthorI 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:
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 hroncokmannequinMannequinAuthorGit bisect:
c314e60 is the first new commit
commit c314e60
Author: Ethan Furman <ethan@stoneleaf.us>
Date: Tue Jan 12 23:47:57 2021 -0800bpo-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.rstThe code for that
__str__seems very inefficient -- why doesn't it just do:return self.name
?
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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 hroncokmannequinMannequinAuthorThanks. In the meantime, I've opened mesonbuild/meson#8318
bakerdylanc commented
on Feb 9, 2021 bakerdylancmannequinMannequinAuthor 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 hroncokmannequinMannequinAuthorEthan, 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.
Dylan, it's not the
from_str()method, but the__str__method that is the problem. Instead ofdef __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 hroncokmannequinMannequinAuthor"Wait for the warning to appear in at least two major Python versions. It's fine to wait more than two releases."
https://www.python.org/dev/peps/pep-0387/#basic-policy-for-backwards-compatibility
So indeed, if you add the warning in 3.10, the behavior can be removed from 3.12 earliest.
DeprecationWarning will be active in 3.10 and 3.11 with removal in 3.12.
hroncok commented
on Mar 3, 2021 hroncokmannequinMannequinAuthorThank you, Ethan.
You're welcome. Thank you for pushing the issue! :-)
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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