problem with ABCMeta.__prepare__ when called after types.new_class

[Ricyteach]

BPO	33190
Nosy	@ilevkivskyi, @Ricyteach
PRs	bpo-33190: elucidated types.new_class docstring #6317
Files	abcmeta_prepare.py: demo of 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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GitHub fields:

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://proxy-ga.blitzz.co/proxy/123456/github.com/Ricyteach'

bugs.python.org fields:

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://proxy-ga.blitzz.co/proxy/123456/bugs.python.org/issue33190'
versions = ['Python 3.7', 'Python 3.8']

[Ricyteach]

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.

[ilevkivskyi]

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.

[Ricyteach]

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.

[ilevkivskyi]

https://proxy-ga.blitzz.co/proxy/123456/devguide.python.org/ will help you.

[Ricyteach]

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

[furkanonder]

@ilevkivskyi I think we can close the problem because the issue is not a bug, it is a misunderstanding.

[ilevkivskyi]

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