[Cython] [Python-Dev] C-level duck typing

mark florisson markflorisson88 at gmail.com
Thu May 17 11:26:41 CEST 2012

On 17 May 2012 08:36, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Dag Sverre Seljebotn, 17.05.2012 09:12:
>> Stefan Behnel wrote:
>>> mark florisson, 16.05.2012 21:49:
>>>> On 16 May 2012 20:15, Stefan Behnel wrote:
>>>>> Why not just use a custom attribute on callables that hold a
>>>>> PyCapsule? Whenever we see inside of a Cython implemented function
>>>>> that an object variable that was retrieved from the outside,
>>>>> either as a function argument or as the result of a function call,
>>>>> is being called, we try to unpack a C function pointer from it on
>>>>> all assignments to the variable. If that works, we can scan for a
>>>>> suitable signature (either right away or lazily on first access)
>>>>> and cache that. On each subsequent call through that variable,
>>>>> the cached C function will be used.
>>>>> That means we'd replace Python variables that are being called by
>>>>> multiple local variables, one that holds the object and one for each C
>>>>> function with a different signature that it is being called with. We
>>>>> set the C function variables to NULL when the Python function variable
>>>>> is being assigned to.
>>>>> When the C function variable is NULL on call, we scan for a matching
>>>>> signature and assign it to the variable.  When no matching signature
>>>>> can be found, we set it to (void*)-1.
>>>>> Additionally, we allow explicit user casts of Python objects to C
>>>>> function types, which would then try to unpack the C function, raising
>>>>> a TypeError on mismatch.
>>>>> Assignments to callable variables can be expected to occur much less
>>>>> frequently than calls to them, so this will give us a good trade-off
>>>>> in most cases. I don't see why this kind of caching would be any slower
>>>>> inside of loops than what we were discussing so far.
>>>> This works really well for local variables, but for globals, def
>>>> methods or callbacks as attributes, this won't work so well, as they
>>>> may be rebound at any time outside of the module scope.
>>> Only half true for globals, which can be declared "cdef object", e.g.
>>> for imported names. That would allow Cython to see all possible
>>> reassignments in a module, which would then apply the above scheme.
>>> I don't think def methods are a use case for this because you'd either
>>> cpdef them or even cdef them if you want speed. If you want them to be
>>> overridable, you'll have to live with the speed penalty that that
>>> implies.
>>> For object attributes, you have to pay the penalty of a lookup anyway,
>>> no way around that. We can't even cache anything here (e.g. with a
>>> borrowed reference) because the attribute may be rebound to another
>>> object that happens to live at the same address as the previous one.
>>> However, if you want speed, you'd do it as in CPython and assign the
>>> object to a local variable to pay the lookup of only once. Problem
>>> solved.
>> 'Problem solved' by pushing the work over to the user? By that line
>> of argument, why not just kill of Cython and require users to write C?
> What part of the work does the above proposal push to the user? To make it
> explicit that an object attribute or a global variable is not expected to
> change during whatever a loop does? Well, yes. If the user knows that, a
> global cdef or an assignment to a local variable is the easiest, safest,
> fastest and most obvious way to tell Cython that it should take advantage
> of it. Why invent yet another declaration for this?
>> Hyperbole aside; do you really believe it is worth dropping a relatively
>> easy optimization just to make the C level code more to the taste of
>> some python-dev posters?
> I find the above much easier for all sides. It's easier to implement for us
> and others, it doesn't have any impact on CPython and I also find it easier
> to understand for users.
> Besides, I was only responding to Mark's remarks (pun not intended) about
> the few cases where this may not immediately yield the expected advantage.
> They are easy to fix, that's all I was saying. In most cases, this simple
> scheme will do the right thing without any user interaction, and it does
> not require any changes or future constraints on CPython.
> So, why not just implement this for now and *then* re-evaluate if we really
> need more, and if we can really do better?
> Stefan
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Hm, I think we should implement fast dispatch first, and if an
additional optimization with hoisted function pointer unpacking leads
to non-negligible performance gains, we can just implement both. I
don't think python-dev cares much about C-level interfaces, and Martin
is right that we can just do the same thing through metaclasses, which
would be portable across versions and just as fast (probably :).

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