struct.Struct inheritance with Python 3.12.0

[livrrr]

Bug report

Bug description:

import struct

class MyStruct(struct.Struct):
    def __init__(self):
        super().__init__('>h')

obj = MyStruct()

When I run this code I receive an error:

Traceback (most recent call last):
  File "/home/user/bug.py", line 7, in <module>
    obj = MyStruct()
          ^^^^^^^^^^
TypeError: Struct() missing required argument 'format' (pos 1)

It is rather strange error, I have passed format parameter to base class constructor, I receive this error with any value of format parameter.
There are no any problems with this code in Python 3.11 and older.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-112358: Fix Python 3.12 regression with subclassing struct.Struct. #112424
• [3.12] gh-112358: Fix Python 3.12 regression with subclassing struct.Struct (GH-112424) #112426

[Eclips4]

bisected to c8c0afc
cc @kumaraditya303

[AlexWaygood]

Looks like another user also hit this problem, but reported it on the PR rather than opening an issue: #94532 (comment)

[mdickinson]

Hmm, this isn't nice. I'm not seeing any good options here other than simply reverting the offending commit (and re-opening the issues that PR #94532 was supposed to close). @kumaraditya303?

[mdickinson]

Hmm, this isn't nice. [...]

To elaborate: the existence of struct.Struct subclasses that extend the __init__ method means that Struct.__new__ can make essentially no assumptions about the arguments it gets. It might get no arguments; it might get just the format, it might get the format plus additional things, or it might get a collection of arguments that doesn't include the format at all. Previously all those things worked because Struct.__new__ was simply object.__new__, which accepted any combination of args and kwargs.

This makes the solution in #94532 of initialising the struct in Struct.__new__ non-viable - at least, not without backwards compatibility breakage, which would have to be managed in the usual way (deprecation period, etc.).

I've opened #112424 for reversion. Once merged, it'll need to be backported to 3.12, and issues #75960 and #78724 will need to be re-opened.