Finding lines with length of 0.0 inside polygon with Python script in QGIS

Question

I have a vector layer (PostGIS) with line geometries. Some of these geometries have a length of 0.0, but they have two points with the same coordinate. I want to handle this features within a python script and tried to fetch all of them inside a polygon. When I use a QgsFeatureRequest,

feats = layer.getFeatures(QgsFeatureRequest().setFilterRect(myPolygon.boundingBox()))

the result contains only features with a length > 0.0, but not "zero-length-lines". How can I fetch these lines, too?

In the QGIS application I can see those "zero-length-lines"and I can select them manually. I checked this with a small script:

layer = iface.activeLayer()
fs = layer.selectedFeatures()
for f in fs:
 g = f.geometry()
 print("WKB type",g.type())
 if g.type() == QgsWkbTypes.LineGeometry:
 print("LineGeom")
 print("length: ", g.length())

It returns for the selected features:

WKB type 1
LineGeom
length: 0.0 <----
WKB type 1
LineGeom
length: 2.996556190491762

• Ian Turton: loop through the features and check the length
• Spacedman: I've just made a layer with two features in a unit square, one of which is a zero-length line, and something like your first bit of code, feats = layer.getFeatures(QgsFeatureRequest().setFilterRect(QgsRectangle(-1, -1, 2, 2))) gets both of them. Are you sure your mypolygon contains everything?
• Spacedman: @IanTurton the Q assertion is that when got with a setFilterRect on a polygon the zero-length features aren't returned, so can't be looped over. But my little test seems to imply they are returned (at least with a QgsRectangle query, which should be equivalent to a bounding box). Something's not right...
• user30184: Such linestrings are invalid. Example: select ST_IsValid('LINESTRING(1 1, 1 1)') returns "false". Would you like to find them for fixing the geometries or why?
• Peter: Thanks for your help! QgsFeatureRequest.GeometryNoCheck was it ... Yes, I want to make valid geometries from this lines (the second point should get a new coordinate depending on other geometries.

Answer 1

To also get invalid geometries you can try to set the QgsFeatureRequest() to GeometryNoCheck actively:

# just get some test features...
lines = QgsProject.instance().mapLayersByName('lines')[0]
polygons = QgsProject.instance().mapLayersByName('polygon')[0]
mypolygon = polygons.getFeature(1)
# setup the QgsFeatureRequest():
req = QgsFeatureRequest()
req.setFilterRect(mypolygon.geometry().boundingBox())
req.setInvalidGeometryCheck(QgsFeatureRequest.GeometryNoCheck)
# get the features and iterate over them, it should also print length 0 of the invalid ones
linefeatures = lines.getFeatures(req)
for linefeature in linefeatures:
 print(linefeature.geometry().length())

Other attributes than GeometryNoCheck would be GeometryAbortOnInvalid or GeometrySkipInvalid, see https://qgis.org/pyqgis/3.0/core/Feature/QgsFeatureRequest.html. I guess you may have set GeometrySkipInvalid as default in your settings or something like that.