Bug report

Bug description:

The Struct constructor permits creation of half-initialized Struct's, e.g.:

>>>from _structimport Struct>>> s= Struct.__new__(Struct)>>> s.unpack_from(b'boo!')# this might be a crash!Traceback (most recent call last):  File "<python-input-2>", line 1, in <module>    s.unpack_from(b'boo!')    ~~~~~~~~~~~~~^^^^^^^^^SystemError: Objects/tupleobject.c:40: bad argument to internal function>>> s= Struct.__new__(Struct,1,2,3)# anything is accepted>>> s.unpack_from(b'boo!')Traceback (most recent call last):  File "<python-input-6>", line 1, in <module>    s.unpack_from(b'boo!')    ~~~~~~~~~~~~~^^^^^^^^^SystemError: Objects/tupleobject.c:40: bad argument to internal function

c.f.:

>>> int.__new__(int, 1, 2, 3)Traceback (most recent call last):  File "<python-input-7>", line 1, in <module>    int.__new__(int, 1, 2,3 )    ~~~~~~~~~~~^^^^^^^^^^^^^^TypeError: int() takes at most 2 arguments (3 given)

TheStruct.__new__() dunder handles only memory allocation, the rest goes to theStruct.__init__(). That doesn't make sense for immutable type (whichStruct() pretend to be in fact) and introduce a number of issues, e.g.:

• _struct.Struct: calling functions without calling __init__ results in SystemError #78724
• assertion failures and a crash when using an uninitialized struct.Struct object #75960
• Use-after-free ins_pack_internal via re-entrant__bool__ #143379

The proper way to fix all this, probably, is moving all initialization logic to theStruct.__new__() dunder:

It is more convenient to initialize the Struct instance in__new__ than in__init__, and it makes sense, since Struct instances are cached and therefore can be considered immutable like ints or tuples. But the possibility of creating subclasses and the existence of subclasses in the wild makes this a breaking change.

Originally posted by@serhiy-storchaka in#112358

Fromdocs:

A good rule of thumb is that for immutable types, all initialization should take place intp_new, while for mutable types, most initialization should be deferred totp_init.

This was done in#94532, which then was reverted due to introduced breackage (#112358).

I propose:

1. deprecate repeated calls of theStruct.__init__() on initialized Struct (will be a no-op eventually)
2. move all initialization logic toStruct.__new__(), makeself.__init__() a no-op if__new__() got one argument
3. deprecate calls ofStruct.__new__() without required argument.

TheStruct.__init__() dunder method will be removed in the CPython 3.20. I suggest to close all opened referenced above issues as duplicates of this one.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-143715: deprecate incomplete initialization of struct.Struct() #143659