[GNC-dev] Python-Bindings: issue when C expects GList argument

[john]

> On May 10, 2022, at 1:53 AM, Matteo Lasagni <laser820 at gmail.com> wrote:
> 
> Thank you, John!
> 
> I fixed it by adding the following into base-typemaps.i:
> 
> %typemap(in) GList * {
>     $1 = NULL;
>     /* Check if is a list */
>     if (PyList_Check($input)) {
>         int i;
>         int size = PyList_Size($input);
>         for (i = size-1; i >= 0; i--) {
>             PyObject *o = PyList_GetItem($input, i);
>             $1 = g_list_prepend($1, o);
>         }
>     } else {
>         PyErr_SetString(PyExc_TypeError, "not a list");
>         return NULL;
>     }
> }
> 
> I had overlooked the swig definition in engine.i, where there is a typemap (in) for GList, but then it is cleared right afterwards...
> I am not (yet) very familiar with swig :-)
> 
> I have also noticed that CreatePaymentLotSecs does not work out of the box, as it expects **Transaction while only *Transaction is type mapped.
> 
> I will test these changes then I will figure out how to share them with the developers.
> 
> 
> Matteo
> 
> 
> On Tue, 10 May 2022 at 02:32, John Ralls <jralls at ceridwen.us <mailto:jralls at ceridwen.us>> wrote:
> 
> 
> > On May 9, 2022, at 2:17 PM, Matteo Lasagni <laser820 at gmail.com <mailto:laser820 at gmail.com>> wrote:
> > 
> > Hi,
> > I am writing a python script to automatically associate a set of
> > invoices to a single transaction.
> > To this end, I am trying to use the function AutoApplyPaymentsWithLots
> > with no success:
> > 
> > 
> > lots = [lot1, lot2, lotn, ..]
> > owner.AutoApplyPaymentsWithLots(lots)
> > 
> > This causes the following error:
> > 
> > TypeError: in method 'gncOwnerAutoApplyPaymentsWithLots', argument 2 of
> > type 'GList *'
> > 
> > I have tried different solutions from passing different types of python
> > objects, to updating the swig typemap and also to modifying the
> > auto-generated gnucash_core.c...nothing worked out so far.
> > I could write an additional c-wrapper, but I am sure there is a more
> > general solution.
> 
> The fundamental problem is that there's only a Python out typemap for GList, that is one that converts from a GList* to a python list. Lacking an in typemap the gncOwner.AutoApplyPaymentsWithLots python wrapper expects an already constructed and Swig-wrapped GList*.
> 
> Regards,
> John Ralls
> 
> 


Nice. Would you put that in a pull request?

There are two typemap(in) GList* in engine.i, but they're both for Guile (Scheme), not Python. 'scm' is the giveaway.

If there's a way to implement an output pointer parameter in Python I don't know about it. You can pass in a container object and put the output Transaction* in the container, but that's both a bit of a hack and doesn't have the same meaning as Transaction** nor is it very pythonic. Nor, for that matter, is gncOwnerCreatePaymentLotSecs a very good design as it mixes responsibilities. Unfortunately there's a lot of that in GnuCash because most of it is written with a very narrow focus, in this case providing for the payments dialog.

Regards,
John Ralls