Description

assignee = None
closed_at = None
created_at = <Date 2018-03-30.20:43:02.783>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'problem with ABCMeta.__prepare__ when called after types.new_class'
updated_at = <Date 2018-03-31.02:58:22.949>
user = 'https://github.com/Ricyteach'

activity = <Date 2018-03-31.02:58:22.949>
actor = 'Ricyteach'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-03-30.20:43:02.783>
creator = 'Ricyteach'
dependencies = []
files = ['47509']
hgrepos = []
issue_num = 33190
keywords = ['patch']
message_count = 5.0
messages = ['314714', '314716', '314717', '314718', '314731']
nosy_count = 2.0
nosy_names = ['levkivskyi', 'Ricyteach']
pr_nums = ['6317']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33190'
versions = ['Python 3.7', 'Python 3.8']

Activity

1. Ricyteach commented on Mar 30, 2018

   

   Ricyteachmannequin

   on Mar 30, 2018

   MannequinAuthor

   More actions

   I am pretty sure this is a bug. If not I apologize.

   Say I want to dynamically create a new C class, with base class MyABC (and dynamically assigned abstract method m). This works fine if I use type, but if I use new_class, the keyword argument to the m method implementation gets lost somewhere in the call to ABCMeta.__prepare___.

   I've attached a file to demo. Thanks.
2. Ricyteach

   added

   3.7 (EOL)end of lifeend of life

   3.8 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Mar 30, 2018
3. ilevkivskyi commented on Mar 30, 2018

   

   ilevkivskyi

   on Mar 30, 2018

   Member

   More actions

   This is not a bug, but a misunderstanding:

   • First, ABCMeta doesn't have __prepare__, it is just type.__prepare__
   • Second, the third argument to types.new_class is called kwds for a reason. It is not a namespace like in type but the set of keywords in the equivalent class definition. For example:

   types.new_class('Test', (A, B), {'metaclass': Meta, 'foo': 42})

   id equivalent to

   class Test(A, B, metaclass=Meta, foo=42):
       pass

   If you want to populate the namespace, then you should use the fourth argument exec_body which should be a callable that takes newly created dictionary and populates it with items. For your case it should be:

   C = new_class("C", (MyABC,), {}, exec_body=lambda ns: ns.update(namespace))

   If you want to clarify the corresponding docstring, then please open a PR. Otherwise you can close the issue.
4. Ricyteach commented on Mar 30, 2018

   

   Ricyteachmannequin

   on Mar 30, 2018

   MannequinAuthor

   More actions

   Excellent; thank you very much for the explanation.

   I have never done a PR on a project this size but I'll give it a try.
5. ilevkivskyi commented on Mar 30, 2018

   

   ilevkivskyi

   on Mar 30, 2018

   Member

   More actions

   https://devguide.python.org/ will help you.
6. Ricyteach commented on Mar 31, 2018

   

   Ricyteachmannequin

   on Mar 31, 2018

   MannequinAuthor

   More actions

   Thank you; I gave it a go. Hopefully didn't cause too much additional work for someone.
7. ezio-melotti

   transferred this issue fromon Apr 10, 2022
8. furkanonder commented on May 13, 2023

   

   furkanonder

   on May 13, 2023

   Contributor

   More actions

   @ilevkivskyi I think we can close the problem because the issue is not a bug, it is a misunderstanding.
9. ilevkivskyi commented on May 13, 2023

   

   ilevkivskyi

   on May 13, 2023

   Member

   More actions

   There is still an option to improve the docstring for new_class() but if no-one wants to do this, then we can close.
10. hugovk

    added

    docsDocumentation in the Doc dirDocumentation in the Doc dir

    easy

    pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

    and removed

    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

    on Oct 26, 2026
11. github-project-automation

    added this to docs issueson Oct 26, 2026
12. github-project-automation

    moved this to Todo in docs issueson Oct 26, 2026

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