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[Python-Dev] Can we use "designated initializer" widely in core modules?

Victor Stinner victor.stinner at gmail.com
Wed Jan 18 09:52:48 EST 2017 
2017-01-18 12:33 GMT+01:00 Antoine Pitrou <solipsis at pitrou.net>:
> I don't think I thought about that idea at the time.  tp_finalize
> doesn't benefit many extension types, so it's not a huge burden to add
> a FLAGS value for the few cases where you want to use it.

It isn't obvious to me that I have to define explicitly a
Py_TPFLAGS_HAVE_FINALIZE flag if I want to _inherit_ the finalizer
from the base class. The flag requires an #ifdef to only define it on
Python >= 3.4, which can be painful when you have to support older
Python versions and want a single code base for all Python versions.

If a C extension defines a type which inherit from a type which has a
finalizer, currently the finalizer is not inherited if the new type
doesn't set explicitly the Py_TPFLAGS_HAVE_FINALIZE flag.

I checked the _socket.socket type. Hopefully, sock_dealloc() calls
PyObject_CallFinalizerFromDealloc(), so I guess that it should "just
work" if tp_finalize is not inherited in the tp_finalize slot.

But for tp_fastcall, if a type doesn't have the
Py_TPFLAGS_HAVE_FASTCALL flag, the fastcall_wrapper() has to find the
first base class with a tp_fastcall slot which. The search has a
complexity of O(n), so it has an impact on performance (even if the
impact is low).

The flag only says that the C structure contains the field, not that
it's set. So I think that it's safe to add Py_TPFLAGS_HAVE_FINALIZE
and Py_TPFLAGS_HAVE_FASTCALL to Py_TPFLAGS_DEFAULT.

Victor
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