[Enum] Exception being ignored in custom datatypes

import enum

class Base:
    def __init__(self, x):
        print('In Base init')
        raise ValueError("I don't like", x)

class MyEnum(Base, enum.Enum):
    A = 'a'
    def __init__(self, y):
        print('In MyEnum init')
        self.y = y

MyEnum should not be created because of the exception in Base.__init__.

Linked PRs

gh-102549: [Enum] fail enum creation when data type raises in __init__ #103149
[3.11] gh-102549: [Enum] fail enum creation when data type raises in __init__ (GH-103149) #103154

Just tested this - MyEnum is indeed not created and the ValueError is raised.

import enum

class Base:
    def __init__(self, x):
        print('In Base init')
        raise ValueError("I don't like", x)

class MyEnum(Base, enum.Enum):
    A = 'a'
    def __init__(self, y):
        print('In MyEnum init')
        self.y = y

MyEnum(1)

Output

In Base init
In MyEnum init
Traceback (most recent call last):
  File "/Users/tnie/code/cpython/tmp.py", line 14, in <module>
    MyEnum(1)
  File "/Users/tnie/code/cpython/Lib/enum.py", line 728, in __call__
    return cls.__new__(cls, value)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tnie/code/cpython/Lib/enum.py", line 1132, in __new__
    raise ve_exc
ValueError: 1 is not a valid MyEnum

What behaviour are you describing that's a bug?

MyEnum should not have been created in the first place. The behavior in the patch I'm working on:

import enum

class Base:
    def __init__(self, x):
        print('In Base init')
        raise ValueError("I don't like", x)

class MyEnum(Base, enum.Enum):
    A = 'a'
    def __init__(self, y):
        print('In MyEnum init')
        self.y = y

and

In Base init
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/source/python/cpython/Lib/enum.py", line 561, in __new__
    raise exc
  File "/source/python/cpython/Lib/enum.py", line 278, in __set_name__
    raise TypeError(
TypeError: _value_ not set in __new__, unable to create it
>>> MyEnum
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'MyEnum' is not defined

pythongh-102549: [Enum] fail enum creation when data type raises in _…

gh-102549: [Enum] fail enum creation when data type raises in __init__ (

pythongh-102549: [Enum] fail enum creation when data type raises in _…

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[Enum] Exception being ignored in custom datatypes #102549

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[Enum] Exception being ignored in custom datatypes #102549

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kwsp commented on Mar 9, 2023

ethanfurman commented on Mar 9, 2023

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