[Python-Dev] When does `PyType_Type.tp_alloc get assigned to PyType_GenericAlloc ?
Randy Eels
randyeels at gmail.com
Wed Feb 17 17:24:02 EST 2016
On Sun, Feb 7, 2016 at 8:45 PM, Guido van Rossum <guido at python.org> wrote:
> I think it's probably line 2649 in typeobject.c, in type_new():
>
> type->tp_alloc = PyType_GenericAlloc;
I pondered it but it doesn't seem to be that. Isn't `type_new` called
*after* PyType_Type.tp_alloc has been called? I thought that line was only
being executed for user-defined types, and maybe some built-in types, but
certainly not PyType_Type.
On Sun, Feb 7, 2016 at 9:27 PM, eryk sun <eryksun at gmail.com> wrote:
> On Sun, Feb 7, 2016 at 7:58 AM, Randy Eels <randyeels at gmail.com> wrote:
> >
> > Yet, I can't seem to understand where and when does the `tp_alloc` slot
> of
> > PyType_Type get re-assigned to PyType_GenericAlloc. Does that even
> happen?
> > Or am I missing something bigger?
>
> _Py_InitializeEx_Private in Python/pylifecycle.c calls _Py_ReadyTypes
> in Objects/object.c. This calls PyType_Ready(&PyType_Type) in
> Objects/typeobject.c, which assigns type->tp_base = &PyBaseObject_Type
> and then calls inherit_slots. This executes COPYSLOT(tp_alloc), which
> assigns PyType_Type.tp_alloc = PyBaseObject_Type.tp_alloc, which is
> statically assigned as PyType_GenericAlloc.
>
> Debug trace on Windows:
>
> 0:000> bp python35!PyType_Ready
> 0:000> g
> Breakpoint 0 hit
> python35!PyType_Ready:
> 00000000`6502d160 4053 push rbx
> 0:000> ?? ((PyTypeObject *)@rcx)->tp_name
> char * 0x00000000`650e4044
> "object"
> 0:000> g
> Breakpoint 0 hit
> python35!PyType_Ready:
> 00000000`6502d160 4053 push rbx
> 0:000> ?? ((PyTypeObject *)@rcx)->tp_name
> char * 0x00000000`651d8e5c
> "type"
> 0:000> bp python35!inherit_slots
> 0:000> g
> Breakpoint 1 hit
> python35!inherit_slots:
> 00000000`6502c440 48895c2408 mov qword ptr [rsp+8],rbx
> ss:00000000`0028f960={
> python35!PyType_Type
> (00000000`6527cba0)}
>
> At entry to inherit_slots, PyType_Type.tp_alloc is NULL:
>
> 0:000> ?? python35!PyType_Type.tp_alloc
> <function> * 0x00000000`00000000
> 0:000> pt
> python35!inherit_slots+0xd17:
> 00000000`6502d157 c3 ret
>
> At exit it's set to PyType_GenericAlloc:
>
> 0:000> ?? python35!PyType_Type.tp_alloc
> <function> * 0x00000000`65025580
> 0:000> ln 65025580
> (00000000`65025580) python35!PyType_GenericAlloc |
> (00000000`650256a0) python35!PyType_GenericNew
> Exact matches:
> python35!PyType_GenericAlloc (void)
>
This makes quite a bit of sense. I completely overlooked the interpreter
init routines.
Thank you both Guido and eryk!
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