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Able to subclass enum with members by using multiple inheritance #85689

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talsuk5 mannequin opened this issue Aug 10, 2020 · 12 comments
Closed

Able to subclass enum with members by using multiple inheritance #85689

talsuk5 mannequin opened this issue Aug 10, 2020 · 12 comments
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@ethanfurman
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3.8 only security fixes 3.9 only security fixes 3.10 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@talsuk5
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talsuk5 mannequin commented Aug 10, 2020

BPO 41517
Nosy @warsaw, @ericvsmith, @stevendaprano, @ethanfurman, @miss-islington, @websurfer5
PRs
  • bpo-41517: do not allow Enums to be extended #22271
  • [3.9] bpo-41517: do not allow Enums to be extended (GH-22271) #22278
  • [3.8] bpo-41517: do not allow Enums to be extended (GH-22271) #22279
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    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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    GitHub fields:

    assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2020-09-16.14:43:14.465>
created_at = <Date 2020-08-10.17:47:58.005>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'Able to subclass enum with members by using multiple inheritance'
updated_at = <Date 2020-09-16.14:43:14.465>
user = 'https://bugs.python.org/talsuk5'

    bugs.python.org fields:

    activity = <Date 2020-09-16.14:43:14.465>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-09-16.14:43:14.465>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2020-08-10.17:47:58.005>
creator = 'talsuk5'
dependencies = []
files = []
hgrepos = []
issue_num = 41517
keywords = ['patch']
message_count = 12.0
messages = ['375133', '375135', '375139', '375141', '375143', '375145', '375147', '375157', '376921', '376994', '376997', '376998']
nosy_count = 8.0
nosy_names = ['barry', 'eric.smith', 'steven.daprano', 'eli.bendersky', 'ethan.furman', 'miss-islington', 'Jeffrey.Kintscher', 'talsuk5']
pr_nums = ['22271', '22278', '22279']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41517'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']
    @talsuk5
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    talsuk5 mannequin commented Aug 10, 2020

    when inheriting an implemented enum, we get a runtime error
    But when creating a multiple inheritance like the following, it works… so something feels broken in the enum mechanism

    ------------------------------------

    from enum import IntEnum, Enum

class A(IntEnum):
   a = 1

class B(A, Enum):
   b= 1

print(B.b)

    @talsuk5 talsuk5 mannequin added 3.7 (EOL) end of life stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Aug 10, 2020
    @ericvsmith
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    Could you provide code which demonstrates the problem?

    @talsuk5
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    talsuk5 mannequin commented Aug 10, 2020

    Eric V. Smith it's in the first comment

    @ericvsmith
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    With Python 3.7.4, which is all I have handy, that code does not give a runtime error. It prints "B.b".

    Your text says code "when creating a multiple inheritance like the following, it works". It sounds like you know the code sample works. So I don't understand what code gives a runtime error. What type of error are you seeing?

    @talsuk5
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    talsuk5 mannequin commented Aug 10, 2020

    sorry, I'll explain. by "when inheriting an implemented enum, we get a runtime error" I refer to this https://docs.python.org/3/library/enum.html#restricted-enum-subclassing

    You shouldn't be able to subclass an enum, yet I just showed a loophole. my example code shows you can subclass an enum by using a trick, multiple inheritance.

    I hope it's more clear now :)

    btw it also works with regular Enum inheritance (I just used IntEnum for simplicity)

    @ethanfurman
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    The problem is that class B should raise an error as class A already has members.

    @talsuk5
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    talsuk5 mannequin commented Aug 10, 2020

    Ethan Furman bullseye! thanks for clarifying

    @stevendaprano
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    The documentation says:

    "Allowing subclassing of enums that define members would lead to a violation of some important invariants of types and instances."

    but it isn't clear what those invariants are, or why it is more of a problem for enums than any other subclassing situation. Could the docs be updated to explain why it is prohibited?

    @stevendaprano stevendaprano changed the title Enum multiple inheritance loophole Able to subclass enum with members by using multiple inheritance Aug 10, 2020
    @stevendaprano stevendaprano changed the title Enum multiple inheritance loophole Able to subclass enum with members by using multiple inheritance Aug 10, 2020
    @ethanfurman
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    To answer the invariant question, see this post by Guido:

    https://mail.python.org/pipermail/python-dev/2013-April/125716.html

    @ethanfurman ethanfurman added 3.8 only security fixes 3.9 only security fixes 3.10 only security fixes and removed 3.7 (EOL) end of life labels Sep 16, 2020
    @ethanfurman
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    New changeset 3064dbf by Ethan Furman in branch 'master':
    bpo-41517: do not allow Enums to be extended (bpo-22271)
    3064dbf

    @miss-islington
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    New changeset 3f40121 by Miss Islington (bot) in branch '3.8':
    bpo-41517: do not allow Enums to be extended (GH-22271)
    3f40121

    @miss-islington
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    New changeset 48f9925 by Miss Islington (bot) in branch '3.9':
    bpo-41517: do not allow Enums to be extended (GH-22271)
    48f9925

    @ethanfurman ethanfurman self-assigned this Sep 16, 2020
    @ethanfurman ethanfurman self-assigned this Sep 16, 2020
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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    @ethanfurman ethanfurman

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