Patch-adding __init__ in a class takes no effect when called from a subinterpreter

[hroncok]

BPO	46034
Nosy	@vstinner, @ericsnowcurrently, @hroncok

^{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 = <Date 2021-12-10.12:29:40.628>
created_at = <Date 2021-12-10.12:01:00.299>
labels = ['expert-subinterpreters', 'type-bug', '3.10', '3.11']
title = 'Patch-adding __init__ in a class takes no effect when called from a subinterpreter'
updated_at = <Date 2021-12-10.12:29:40.628>
user = 'https://github.com/hroncok'

bugs.python.org fields:

activity = <Date 2021-12-10.12:29:40.628>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-10.12:29:40.628>
closer = 'vstinner'
components = ['Subinterpreters']
creation = <Date 2021-12-10.12:01:00.299>
creator = 'hroncok'
dependencies = []
files = []
hgrepos = []
issue_num = 46034
keywords = []
message_count = 3.0
messages = ['408196', '408199', '408200']
nosy_count = 3.0
nosy_names = ['vstinner', 'eric.snow', 'hroncok']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46034'
versions = ['Python 3.10', 'Python 3.11']

[hroncok]

Hello,

based on some bug reports reported in:

• Decorators on classes ignored GrahamDumpleton/mod_wsgi#729
• (Py3.10) Error on loading: TypeError: TypeChecker() takes no arguments  poljar/weechat-matrix#293
• https://bugzilla.redhat.com/show_bug.cgi?id=2030621

I have isolated the following reproducer that shows a regression in Python 3.10+. It happens at least with 3.10.0, 3.10.1 and 3.11.0a2. It works as expected in Python 3.9.

The following code is valid and works on 3.9, 3.10, 3.11:

=========================================

class Greeting():
    pass

def new_init(inst, name):
    inst.text = f"Hello, {name}\n"

Greeting.__init__ = new_init

Greeting("Miro")

=========================================

But if we run it via a subinterpreter, it breaks on 3.10+:

=========================================

import _testcapi

code = r"""class Greeting():
    pass

def new_init(inst, name):
    inst.text = f"Hello, {name}\n"

Greeting.__init__ = new_init

Greeting("Miro")"""

_testcapi.run_in_subinterp(code)

=========================================

$ python3.9 reproducer.py
(exits cleanly)

$ python3.10 reproducer.py 
Traceback (most recent call last):
  File "<string>", line 9, in <module>
TypeError: Greeting() takes no arguments

$ python3.11 reproducer.py 
Traceback (most recent call last):
  File "<string>", line 9, in <module>
TypeError: Greeting() takes no arguments

My observation also shows that if the Greeting class already had (any) __init__:

=========================================

class Greeting():
    __init__ = ...

=========================================

It works as expected on 3.9, 3.10, 3.11.

In reality, this is most likely to affect classes with decorators, like:

=========================================

import attr

@attr.s
class TestClass:
    foo = attr.ib()

TestClass("Miro")

=========================================

This works standalone, but fails in a subinterpreter:

$ python3.10 reproducer.py 
Traceback (most recent call last):
  File "<string>", line 7, in <module>
TypeError: TestClass() takes no arguments

[hroncok]

git bisect gives:

ea25180

[vstinner]

I mark this issue as a duplicate of bpo-46006. It's not exactly the same issue, but the root cause is the same: in some functions, Python use fast pointer comparisons when two strings are interned, but this now fails if two interned strings belong to two different interpreters.