[Python-Dev] C API for appending to arrays

[Hrvoje Niksic]

Mike Klaas wrote:
> Do you need to append, or are you just looking to create/manipulate an  
> array with a bunch of c-float values?

Mostly real-life examples I've seen of this were creating an array from 
C values obtained from an external source, such as an on-disk file, or 
another process.  The code example was a (simplified and de-C++-ized) 
snippet of actual code.

> I find As{Write/Read}Buffer sufficient for most of these tasks.

They improve things, as shown in the second example, but they're still 
cumbersome to use for appending/initialization of the array.

> (Note that you can  
> get the size of the resulting c array more easily than you are by  
> using PyObject_Length).

Note that AsWriteBuffer always gives you the buffer size anyway -- you 
can't pass bufsize==NULL.  Since I have to call AsWriteBuffer in each 
iteration (because I don't know when the buffer will resize), calling 
PyObject_Length in addition to that doesn't buy much, if anything.

 > I've included some example pyrex  code that populates a new
 > array.array at c speed.
[...]
>      cdef int NA
>      NA = len(W1)
>      W0 = array('d', [colTotal]) * NA

The thing is, when reading values from a file or a general iterator, you 
typically don't know the number of values in advance.  If I did, I would 
probably use an approach similar to yours.

Thanks for the code -- even if it doesn't help in this case, I 
appreciate it as an instructing example of the advanced usage of Pyrex.