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


On Sun, Nov 11, 2018 at 3:19 PM Victor Stinner <vstinner at redhat.com> wrote:

> Le sam. 10 nov. 2018 ? 04:02, Nathaniel Smith <njs at pobox.com> a ?crit :
> > So is it fair to say that your plan is that CPython will always use
> > the current ("old") API internally, and the "new" API will be
> > essentially an abstraction layer, that's designed to let people write
> > C extensions that target the old API, while also being flexible enough
> > to target PyPy and other "new different Python runtimes"?
>
> What we call is "Python C API" is not really an API. I mean, nobody
> tried to sit down and think about a proper API to access Python from
> C. We happened is that we had an implementation of Python written in C
> and it was cool to just "expose everything". By everything, I mean
> "everything". It's hard to find a secret CPython feature not exposed
> or leaked in the C API.
>
> The problem that I'm trying to solve to "fix the C API" to hide
> implementation details, with one constraint: don't create a new
> "Python 4". I don't want to break the backward compatibility without
> having a slow and smooth transition plan. The "old" and new C API must
> be supported in parallel, at least in the standard CPython,
> /usr/bin/python3.
>
> Writing a new API from scratch is nice, but it's harder to moving all
> existing C extensions from the "old" C API to a new one.
>
> Replacing macros with functions has little impact on backward
> compatibility. Most C extensions should still work if macros become
> functions.
>
> I'm not sure yet how far we should go towards a perfect API which
> doesn't leak everything. We have to move slowly, and make sure that we
> don't break major C extensions. We need to write tools to fully
> automate the conversion. If it's not possible, maybe the whole project
> will fail.
>
> I'm looking for a practical solutions based on the existing C API and
> the existing CPython code base.
>
> > If so, then would it make more sense to develop this as an actual
> > separate abstraction layer? That would have the huge advantage that it
> > could be distributed and versioned separately from CPython, different
> > packages could use different versions of the abstraction layer, PyPy
> > isn't forced to immediately add a bunch of new APIs...
>

I like where this thought is headed!

I didn't investigate this option. But I expect that you will have to
> write a full new API using a different prefix than "Py_". Otherwise,
> I'm not sure how you want to handle PyTuple_GET_ITEM() as a macro on
> one side (Include/tupleobject.h) and PyTuple_GET_ITEM() on the other
> side (hypotetical_new_api.h).
>
> Would it mean to duplicate all functions to get a different prefix?
>

For strict C, yes, namespacing has always been manual collision
avoidance/mangling.  You'd need a new unique name for anything defined as a
function that conflicts with the old C API function names.  You could hide
these from code if you are just reusing the name by using #define in the
new API header of the old name to the new unique name... but that makes
code a real challenge to read and understand at a glance.  Without
examining the header file imports the reader wouldn't know for example if
the code is calling the API that borrows a reference (old api, evil) or one
that gets its own reference (presumed new api).

When things have only ever been macros (Py_INCREF, etc) the name can be
reused if there has never been a function of that name in an old C API.
But beware of reuse for anything where the semantics change to avoid
misunderstandings about behavior from people familiar with the old API or
googling API names to look up behavior.

I suspect optimizing for ease of transition from code written to the
existing C API to the new API by keeping names the same is the wrong thing
to optimize for.

Using entirely new names may actually be a good thing as it makes it
immediately clear which way a given piece of code is written. It'd also be
good for PyObject* the old C API thing be a different type from
PythonHandle* (a new API thing who's name I just made up) such that they
could not be passed around and exchanged for one another without a compiler
complaint.  Code written using both APIs should not be allowed to transit
objects directly between different APIs.

-gps


>
> If you keep the "Py_" prefix, what I would like to ensure is that some
> functions are no longer accessible. How you remove
> PySequence_Fast_GET_ITEM() for example?
>
> For me, it seems simpler to modify CPython headers than starting on
> something new. It seems simpler to choose the proper level of
> compatibility. I start from an API 100% compatible (the current C
> API), and decide what is changed and how.
>
> Victor
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