[QGIS-Developer] Proper way to create polygons in PyQGIS

[Matthias Kuhn]

Hi Hamilton,

It might be just me missing some context, in this case sorry.

What exactly does go wrong?

Would it be possible for you to generate some minimal, self-contained
python code that shows the issue? Like generating one or two memory
layers, plus an additional layer if required, invoke processing.runalg
and provide expected and real results?

This can then be integrated into our test-suite, so we can fix things
and avoid this in the future.

Thanks
Matthias


On 11/01/2017 08:22 PM, C Hamilton wrote:
> It has come to my attention that a few of the plugins that create
> memory layer polygons do not work correctly with the Geoprocessing
> Tools such as Difference. Two examples include the "New Memory Layer"
> plugin (which is being replaced by core) and my own Shape Tools
> plugin. I'm sure that others out there have the same problem. What
> happens is that when you create the polygons with these tools and you
> run the Geoprocessing algorithms such as the Difference operator, you
> get features back that do not contain the proper output polygons.
>
> My question is how do I create these polygons to be valid to the new
> Geoprocessing algorithms? Currently I do the following:
>
> polygonLayer = QgsVectorLayer("Polygon?crs=epsg:4326", "Layer Name",
> "memory")
>
> ppolygon = polygonLayer.dataProvider()
> ...
> featureout = QgsFeature()
>
> featureout.setGeometry(QgsGeometry.fromPolygon([pts]))
>
> # pts is an array of QgsPoints of the polygon vertices
>
>
> ppolygon.addFeatures([featureout])
> ...
> polygonLayer.updateExtents()
> QgsMapLayerRegistry.instance().addMapLayer(polygonLayer)
>
> What needs to be different? One of my concerns is that the
> geoprocessing algorithms takes the data, executes, fails to generate
> proper results, and doesn't give the user any warning that the results
> may be wrong. I sure hope this is only a memory layer issue and not
> also an issue with loading data from files or databases.
>
> Thanks,
>
> Calvin
>
>
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