Awkwardness of C API for making tuples

[John Machin]

Dave Opstad wrote:
> In article <1107289261.185768.116790 at f14g2000cwb.googlegroups.com>,
>  "John Machin" <sjmachin at lexicon.net> wrote:
>
> > What is the purpose of this first loop?
>
> Error handling. If I can't successfully create all the PyInts then I
can
> dispose the ones I've made and not bother making the tuple at all.
> >
> > In what variable-length storage are you storing these (Python)
integers
> > during this first loop? Something you created with (a) PyMem_Malloc
(b)
> > malloc (c) alloca (d) your_own_malloc?
>
> (b) malloc. The sequence here is: 1) malloc; 2) check for malloc
> success; 3) loop to create PyInts (if failure, Py_DECREF those made
so
> far and free the malloc'ed buffer); 4) create new tuple (error checks

> again); and 5) PyTuple_SET_ITEM (no error checks needed)

Don't. If you _must_ allocate your own storage, use PyMem_Malloc.

>
> > 1. Determine the length of the required tuple; this may need a
loop,
> > but only to _count_ the number of C longs that you have.
> > 2. Use PyTuple_New.
> > 3. Loop to fill the tuple, using PyInt_FromLong and
PyTuple_SetItem.
>
> This would certainly be simpler, although I'm not sure I'm as clear
as
> to what happens if, say, in the middle of this loop a PyInt_FromLong
> fails. I know that PyTuple_SetItem steals the reference; does that
mean
> I could just Py_DECREF the tuple and all the pieces will be
> automagically freed? If so, I'll take your recommendation and rework
the
> logic this way.

This is what I believe happens. However even if you did need to do more
cleaning up, you shouldn't penalise the normal case i.e. when
PyInt_FromLong works. The only failure cause AFAIK is running out of
memory.
This should be rare unless it's triggered by your calling malloc :-)