[Python-Dev] When does `PyType_Type.tp_alloc get assigned to PyType_GenericAlloc ?

[Guido van Rossum]

I think it's probably line 2649 in typeobject.c, in type_new():

    type->tp_alloc = PyType_GenericAlloc;

On Sun, Feb 7, 2016 at 5:58 AM, Randy Eels <randyeels at gmail.com> wrote:
> Hi everyone,
>
> I've a question about the implementation of the `type` builtin (in Python
> 3.5).
>
> In Objects/typeobject.c, the `tp_alloc` slot of PyType_Type gets set to 0.
> However, I can see (using gdb) that it later gets assigned to
> `&PyType_GenericAlloc`. I'd argue that this makes sense because, in
> `type_new`, there is a line where that member function gets called without
> previously checking whether that member points to something:
>
> ```
>     /* Allocate the type object */
>     type = (PyTypeObject *)metatype->tp_alloc(metatype, nslots);
> ```
>
> 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?
>
> And, just out of further curiosity, why doesn't the aforementioned slot get
> initialised to `PyType_GenericAlloc` in the first place?
>
> Thanks a lot.
>
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-- 
--Guido van Rossum (python.org/~guido)