[QGIS-Developer] using a features subset as an input for a processing alg

[Nyall Dawson]

On Wed, 11 Sep 2019 at 08:06, G. Allegri <giohappy at gmail.com> wrote:
>
> Dear all,
> I was confident that it would have been simple, while I can’t find the proper way od doing that. Having a reference to a QgsVectorLayer, I would need to use a subset of its features to run an algorithm down the line.
> I hoped there was a way to define an input as afeature source with a filter (QgsFeatureRequest or similar. I’ve tried using QgsProcessingFeatureSourceDefinition with a selection but with no luck:
>
> pointlayer.selectByRect(rect, QgsVectorLayer.SetSelection)
>
> processing.run("qgis:voronoipolygons", {
>          'INPUT': QgsProcessingFeatureSourceDefinition(
>                     pointlayer.id(),
>                     True),
>          'BUFFER': 10,
>          'OUTPUT': outpath})
>
> error, can’t find the layer with that id, since it expects to find it in the proje
>
> Any ideas? Is it possible to build a QgsProcessingParameterDefinition for the task?

It depends if you can get away with a layer subset string for the
filter. (I.e. provider side filter). Because if so, you can embed
these filters in layer sources, e.g.

   processing.run("native:centroids",
{'INPUT':'/home/me/layer.shp|layerid=0|subset=\"something\" =
\'value\'','ALL_PARTS':False,'OUTPUT':'TEMPORARY_OUTPUT'})

Would be nice to extend QgsProcessingFeatureSourceDefinition to
support an expression based filter, that sounds like a good
addition...

Nyall