_struct.Struct should be immutable, missing Py_TPFLAGS_IMMUTABLETYPE flag

[kumaraditya303]

In Python 3.8:

Python 3.8.12 (default, Oct  4 2021, 15:56:52) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import _struct
>>> _struct.Struct.x = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'Struct'
>>> 

On main:

Python 3.12.0a0 (heads/gh-94207-fix-struct-module-leak:9d41baebd0, Jun 25 2022, 07:23:49) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import _struct
>>> _struct.Struct.x=1
>>> _struct.Struct.x
1

This happens because _struct.Struct is missing Py_TPFLAGS_IMMUTABLETYPE in type flags.

cc: @erlend-aasland @encukou

[mdickinson]

_struct.Struct should be immutable

Just wondering: why? Are there benefits to that immutability?

[mdickinson]

Ah, I see there's related discussion at #88074.

[kumaraditya303]

Yes, it was the previous issue ¹ where most of this was fixed but some were left which should be fixed.
cc @gvanrossum

Footnotes

1. https://github.com/python/cpython/issues/88074 ↩

[gvanrossum]

Yeah, please fix.

[erlend-aasland]

Thanks for noticing this, and thanks for the PR!