Fast construction of Python list from large C array of int's - Extension Module

[Alex Martelli]

quadric at primenet.com wrote:

> Hi,
> 
> I'm a bit of a Python newbie and I've written a C extension module. One
> of  the functions must return a Python list whose source is a large (5000
> -
> 6000) C array of integers.  I need a fast way
> of constructing the Python list.  I was planning on something like:
> 
> PyObject * CreateListFromArray( int * array , int array_length)
> {
> PyObject * list = PyList_New( array_length);
> 
> for int i = 0 ; i < array_length ; i++ )
>   {
> PyList_SetItem( list , i , Py_LongFromLong( (long) array[i] )  );
> }
> 
>   return list;
> }
> 
> 
> Is this the fastest way to do this?

I can't think of a faster one, at least not offhand.  Returning other types, 
such as an instance of array.array, might be faster, but if you need a list 
specifically, that's not an option.


> Any inherent dangers here?

Yes: not only pointer 'list', as you realize later, but also EACH pointer 
resulting from PyLong_FromLong COULD be null, with dangerous
results if you don't check.  Do check, and if you get any null, decref
the ones you had previously added and then raise out-of-memory.

This may slow you down by a tiny amount, but risking a nasty crash
is not a viable alternative, IMHO.  You probably won't be able to
_measure_ the proportional slowdown, I suspect.


Alex