[QGIS-Developer] Using memory layer in Processing with SAGA and GRASS

[Nyall Dawson]

On 23 April 2018 at 19:59, matteo <matteo.ghetta at gmail.com> wrote:
> Hi devs,
>
> sorry if I missed something on this topic, but it seems that it is not
> possible using memory layer when running algorithms from the console.
>
> A simple example should explain everything:
>
> # QGIS
> parameters = {
>     'INPUT' : 'big',
>     'OVERLAY': 'small',
>     'OUTPUT':'memory:'
> }
>
> result = processing.run("qgis:difference", parameters)
> QgsProject.instance().addMapLayer(result['OUTPUT'])
>
> # SAGA
>
> parameters_saga = {
>     'A': 'big',
>     'B': 'small',
>     'SPLIT': True,
>     'RESULT': 'memory:'
> }
>
> result_saga = processing.run("saga:difference", parameters_saga)
>
> # nothing is added with this code
> QgsProject.instance().addMapLayer(result_saga['RESULT'])
>
>
> with the code of above I noticed that the big difference is in:
>
> # QGIS
> result['OUTPUT']
> <qgis._core.QgsVectorLayer object at 0x7f92a8a479d8
>
> and
>
> #SAGA
> result_saga['RESULT']
> 'memory:'
>
>
> the same happens with GRASS. The algorithms work fine if the result is
> saved as file
>
> Am I missing something for SAGA and GRASS?

No - neither SAGA nor GRASS have any concept of QGIS memory layers.
You need to give them a disk-based file path to save their outputs to.

If you use a memory layer as an input to these algs it will also have
to save it out to a disk based format, but this is done automatically
by saving to a file in the temp folder. We could potentially do the
same thing to allow SAGA/GRASS algs to output to a memory layer by
first saving the output to a shp in the temp folder, then loading that
file and copying the features to a memory layer. But it would be
rather inefficient and counter productive - you're still being forced
to loop over the disk-based format, so you're just adding extra
processing for little gain.

Nyall