[Pyrex] newbie list processing question

[Greg Ewing]

Daniel Ashbrook wrote:
> I've been trying to figure it out using 
> PyList_GetItem/SetItem and so on, but it seems extremely convoluted, like:
> 
> PyList_SetItem(l,i,PyInt_FromLong(PyInt_AsLong(PyList_GetItem(l,i))+1))

There's no need for it to be *that* convoluted! Pyrex
will take care of all the conversions between C and
Python ints, so you can just write

   PyList_SetItem(l, i, PyList_GetItem(l, i) + 1)

That will actually do the addition as a Python
addition, but that's probably faster than converting
to a C int and back anyway.

To answer your main question, currently, calling
the Python/C API directly is the only way to do any
better than what Pyrex naturally generates.

In the next release, though, you'll be able to declare
things as lists and have some of this done automatically.

Don't expect to get any spectacular improvements from
this, however. Depending on what you're doing, the
slowest part is probably dealing with the Python
ints. If you're doing a lot of arithmetic on the
list items, you may be better off converting the
whole list to an array of C ints, operating on that,
and converting the result back to a Python list.

In other words, there are no magic techniques to
make processing Python lists blazingly fast. The
fact that they're Python lists of Python objects
puts some fundamental limits on what can be
achieved.

> (Plus that line doesn't work - trying to 
> use "for i from" gives me errors about making i an integer from a 
> pointer without a cast.)

Which line? There was no "for i from" anywhere
in the code you included.

-- 
Greg