[Python-ideas] Fwd: [RFC] draft PEP: Dedicated infix operators for matrix multiplication and matrix power

[M.-A. Lemburg]

On 18.03.2014 12:27, Nick Coghlan wrote:
> On 18 March 2014 20:47, Robert Kern <robert.kern at gmail.com> wrote:
>> On 2014-03-18 08:02, Nick Coghlan wrote:
>>>  > operator.matmul and PyObject_MatrixMultiply are obvious enough, but
>>>  > I'm afraid I'm not too clear on the tradeoffs about adding a C level
>>>  > type slot, or even entirely sure what the alternative is. (I guess I
>>>  > just assumed that all special methods used C level type slots and
>>>  > there was nothing to think about.) Do you (or anyone) have any
>>>  > thoughts?
>>>
>>> I suspect you're going to want one, as without it, the implementation
>>> method
>>> ends up in the class dict instead (the context management protocol works
>>> that way).
>>>
>>> I suspect the design we will want is a new struct for Py_Matrix slots
>>> (akin to
>>> those for numbers, etc). The alternative would be to just add more
>>> "Number"
>>> slots, but that isn't really accurate.
> 
> 
> So, here's the change to PyHeapType object that makes the most sense
> to me (assuming both "@" and "@@" are added - drop the new "power"
> methods if "@@" is dropped from the PEP):
> 
> - add "PyMatrixMethods as_matrix;" as a new field in PyHeapTypeObject
> - define PyMatrixMethods as:
> 
>     typedef struct {
>         binaryfunc mt_multiply;
>         binaryfunc mt_power;
>         binaryfunc mt_inplace_multiply;
>         binaryfunc mt_inplace_power;
>    } PyMatrixMethods;
> 
> This approach increases the size of all type objects by one pointer.
> 
> The other way to do it would be to just add four new slots to PyNumberMethods:
> 
>         binaryfunc nb_matrix_multiply;
>         binaryfunc nb_matrix_power;
>         binaryfunc nb_inplace_matrix_multiply;
>         binaryfunc nb_inplace_matrix_power;
> 
> This approach increases the size of all type objects that define one
> or more of the numeric functions by four pointers, and doesn't really
> make sense at a conceptual level. The latter is the main reason I
> prefer the separate PyMatrixMethods struct.

I don't think that it's a good idea to make all type objects
larger just to address matrix multiplications which none of the
builtin types will actually use, so +1 on adding to the number
methods, even if a matrix isn't a number (they still use numbers,
so it's not that far off :-)), but -1 on adding another slot
struct.

Aside: Mechanisms such as namedtuple add lots of new type
objects to the runtime environment, so it's no longer safe to
assume that the size of type objects doesn't really matter much
in real-life applications.

-- 
Marc-Andre Lemburg
eGenix.com

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