In Python 3.8: ```console 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: ```console 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