[Cython] -dagss merge? + what next

[Dag Sverre Seljebotn]

Robert Bradshaw wrote:
> On Jul 29, 2008, at 2:25 AM, Dag Sverre Seljebotn wrote:
> 
>> Those are the big tasks, but there are also a few smaller nice-haves I
>> can go for (a "cython.shape" builtin that allows you to retrieve the
>> shape information of a buffer, and so on).
> 
> I saw you mention shape (and other "magic" functions), and I'm  
> curious. Could you elaborate?

When acquiring a buffer, you get hold of a lot of information that is 
not currently exported -- shape is most important. For NumPy this is ok 
as ndarray exports its shape field for efficient access, but without 
another mechanism you can't write generic algorithms that work with all 
buffers without passing the shape explicitly. This even leads to less 
clean test case code...

I haven't thought much about the list (add as needed) but something like:

- shape(buf) -- Returns a tuple containing the shape of the buffer. 
However, shape(buf)[0] should compile directly to __pyx_bshape0_buf. (I 
thought about shape(buf, 0) as well but decided against, how about you?)

This would be the primary for-loop mechanism:

from cython import shape
...
cdef int i
for i in range(shape(buf)[0]): buf[i] = 9

- buffer(buf), strides(buf), suboffsets(buf) -- Could have these to 
allow low-level access (buffer gets a void* directly, strides and 
suboffsets probably). For contiguous arrays this could be more efficient 
(the proper high-level approach for such things are iterators). The idea 
is that even if you write low-level code, you could use the buffer 
syntax so that you don't need to worry about acquisition.

- ndim(buf) -- If the ndim-at-compile-time-restriction is lifted 
(because you have low-level access or iterators you can use instead of 
indexing) then this will be useful.

Finally, if I get as far as iterators (I won't!), cython.ndenumerate 
could create very efficient iteration code. (Probably a NumPy-specific 
iterator could be less work though...if I can't just copy the approach 
of NumPy iterators directly...NumPy comes with a seperate n-dimensional 
iterator class that allows you to write ndim-generic code.)

-- 
Dag Sverre