Using the WMS-provider.inspect as feature in pyqgis gives: "unable to convert a C++ 'QgsFeatureStoreList' instance to a Python object"

[rduivenvoorde]

plz see:
https://lists.osgeo.org/pipermail/qgis-user/2020-October/046834.html

in short:

• users want to run a script to fire WMS-GetFeatureInfo-requests in pyQGIS
• but the layer.dataProvider().identify in combi with IdentifyFormatFeature returns a result which cannot (yet?) be used in PyQGIS:

TypeError: unable to convert a C++ 'QgsFeatureStoreList' instance to a Python object
or:
cannot unpack non-iterable QgsRasterIdentifyResult object

Is this missing some SIP magic?
Or is this not supposed to work?

Note that
https://docs.qgis.org/3.10/en/docs/pyqgis_developer_cookbook/raster.html#query-values
do not work on a WMS?

To reproduce:

l=qgis.utils.iface.addRasterLayer(
"crs=EPSG:28992&layers=Bebouwingvlak&styles=&format=image/png&url=https://geodata.nationaalgeoregister.nl/kadastralekaart/wms/v4_0?", # uri
"buildings", # name for layer (as seen in QGIS)
"wms" # dataprovider key
)

result = l.dataProvider().identify( QgsPointXY(104606,490213),
             QgsRaster.IdentifyFormatHtml,
             QgsRectangle(104391.1406077263819,490161.0749502293766,104899.6563292678911,490283.3410253541078),
             1465,
             352,
             96)     
print(result.results()[0])  # OK

result = l.dataProvider().identify( QgsPointXY(104606,490213),
             QgsRaster.IdentifyFormatText,
             QgsRectangle(104391.1406077263819,490161.0749502293766,104899.6563292678911,490283.3410253541078),
             1465,
             352,
             96)
print(result.results()[0])  # OK

result = l.dataProvider().identify( QgsPointXY(104606,490213),
             QgsRaster.IdentifyFormatFeature,
             QgsRectangle(104391.1406077263819,490161.0749502293766,104899.6563292678911,490283.3410253541078),
             1465,
             352,
             96)
print(result.results()[0])  # prints: unable to convert a C++ 'QgsFeatureStoreList' instance to a Python object

[agiudiceandrea]

Note that
https://docs.qgis.org/3.10/en/docs/pyqgis_developer_cookbook/raster.html#query-values
do not work on a WMS?

Maybe it is due to the fact that this WMS server does not support the IdentifyValue capability?

[rduivenvoorde]

@agiudiceandrea I am not familiar with the IdentifyValue capability? Is this a new WMS request?
Or do you mean 'getFeatureInfo'? Because that last one does definitly work. I think the problem that the short 'identify' and 'sample' versions do not work because a WMS always needs 'context', as in Width/Height and Bbox params?

[agiudiceandrea]

@rduivenvoorde, with your layer (and AFAIK generally with WMS layers) : print(l.dataProvider().capabilities()) returns 928 (decimal) == 01110100000 (binary) which maps to:
Identify = True // at least one identify format supported
IdentifyValue = False // numerical values
IdentifyText = True // WMS text
IdentifyHtml = True // WMS HTML
IdentifyFeature = True // WMS GML -> feature

QGIS/src/core/raster/qgsrasterinterface.h

Lines 183 to 196 in 4f834ca

	enum Capability
	{
	NoCapabilities = 0,
	Size = 1 << 1, // original data source size (and thus resolution) is known, it is not always available, for example for WMS
	Create = 1 << 2, // create new datasets
	Remove = 1 << 3, // delete datasets
	BuildPyramids = 1 << 4, // supports building of pyramids (overviews)
	Identify = 1 << 5, // at least one identify format supported
	IdentifyValue = 1 << 6, // numerical values
	IdentifyText = 1 << 7, // WMS text
	IdentifyHtml = 1 << 8, // WMS HTML
	IdentifyFeature = 1 << 9, // WMS GML -> feature
	Prefetch = 1 << 10, // allow prefetching of out-of-view images
	};

This means that the WMS raster layer does not support QgsRaster::IdentifyFormatValue format so both identify( , QgsRaster.IdentifyFormatValue) and sample() will fail: QgsError: WMS provider Format not supported

For a WCS layers, print(l.dataProvider().capabilities()) usually returns 98, i.e. Identify = True, IdentifyValue = True.

[rduivenvoorde]

Cool. Thanks Nyall!