[Python-Dev] Experiment an opt-in new C API for Python? (leave current API unchanged)

[Victor Stinner]

Sometimes, code is easier to understand than a long explanation, so
here is a very simple example of modified function for the new C API:

https://bugs.python.org/issue35206
https://github.com/python/cpython/pull/10443/files

PyTuple_GET_ITEM() becomes a function call and the function
implementation checks arguments at runtime if compiled in debug mode.

Technically, the header file still uses a macro, to implicitly cast to
PyObject*, since currently the macro accepts any type, and the new C
API should not change that.

Victor
Le sam. 10 nov. 2018 à 01:53, Victor Stinner <vstinner at redhat.com> a écrit :
>
> To hide all implementation details, I propose to stop using macros and
> use function calls instead. For example, replace:
>
> #define PyTuple_GET_ITEM(op, i) \
>    (((PyTupleObject *)(op))->ob_item[i])
>
> with:
>
> # define PyTuple_GET_ITEM(op, i) PyTuple_GetItem(op, i)
>
> With this change, C extensions using PyTuple_GET_ITEM() does no longer
> dereference PyObject* nor access PyTupleObject.ob_item. For example,
> PyPy doesn't have to convert all tuple items to PyObject, but only
> create one PyObject for the requested item. Another example is that it
> becomes possible to use a "CPython debug runtime" which checks at
> runtime that the first argument is a tuple and that the index is
> valid. For a longer explanation, see the idea of different "Python
> runtimes":
>
>    https://pythoncapi.readthedocs.io/runtimes.html
>
> Replacing macros with function calls is only a first step. It doesn't
> solve the problem of borrowed references for example.
>
> Obviously, such change has a cost on performances. Sadly, I didn't run
> a benchmark yet. At this point, I mostly care about correctness and
> the feasibility of the whole project. I also hope that the new C API
> will allow to implement new optimizations which cannot even be
> imagined today, because of the backward compatibility. The question is
> if the performance balance is positive or not at the all :-)
> Hopefully, there is no urgency to take any decision at this point. The
> whole project is experimental and can be cancelled anytime.
>
> Victor