[Numpy-discussion] On my Cython/NumPy project

[Dag Sverre Seljebotn]

Anne Archibald wrote:
> 2008/6/21 Dag Sverre Seljebotn <dagss at student.matnat.uio.no>:
>> Dag wrote:
>>> General feedback is welcome; in particular, I need more opinions about
>>> what syntax people would like. We seem unable to find something that we
>>> really like; this is the current best candidate (cdef is the way you
>>> declare types on variables in Cython):
>>>
>>> cdef int i = 4, j = 6
>>> cdef np.ndarray[np.float64, 2] arr = np.zeros((10, 10),
>>> dtype=np.float64)
>>> arr[i, j] = 1
>>> ...
>>>
>>
>> Some more important points:
>> - There will likely be a compiler flag (and perhaps per-block or
>> per-variable pragmas) on whether bounds-checking is done or not)
>> - It doesn't look like negative indices will be supported in this mode,
>> as
>> it adds another branch in potentially very tight loops. Opinions on this
>> are welcome though. In safe mode, bounds checking will catch it, while
>> in
>> unsafe mode it will at best segfault and at worst corrupt data.
>>
>> The negative indices thing is potentially confusing to new users. Pex
>> uses
>> a different syntax (arr{i, j} for efficient array lookups) partly to
>> make
>> this fact very explicit. Thoughts?
>
> I am very worried about the negative numbers issue. It's the sort of
> thing that will readily lead to errors, and that produces a
> significant difference between cython and python. I understand the
> performance issues that motivate it, but cython really needs to be
> easy to use or we might as well just use C. As I understand it, the
> ultimate goal should be for users to be able to compile arbitrary
> python code for tiny speed improvements, then add type annotations for
> key variables to obtain large speed improvements. Forbidding negative
> indices now prevents that goal.
>
> My suggestion is this: allow negative indices, accepting the cost in
> tight loops. (If bounds checking is enabled, the cost will be
> negligible anyway.) Provide a #pragma allowing the user to assert that
> a certain piece of code uses no negative indices. Ultimately, the
> cython compiler will often be able to deduce than negative indices
> will not be used within a particular loop and supply the pragma
> itself, but in the meantime this solution allows people to use the
> extremely-convenient negative indices without causing mysterious
> problems, and it sets things up for the future. (Other compiler-based
> cleverness can deal with the most common case by converting explicit
> negative indices.)

Thank you for supplying such useful feedback! It is likely that you have
convinced me.

Dag Sverre