[GIS] Setting QgsExpression for QgsPalLayerSettings in QGIS

pyqgis

How do you apply a QgsExpression to a Label ?

I can do it in QGIS with this simple expression
concat('N:' ,round(Value,0),'\n','Yield:' ,round(Value 2,0))

N: 65

Yield: 40

How do I do it in PyQGIS ? I see a getLabelExpression but no setLabelExpression

This works but I want but only for the field name

 crap = QgsVectorLayer("D:/data.shp", "data", "ogr")
 QgsMapLayerRegistry.instance().addMapLayer(crap)
 palyr = QgsPalLayerSettings()
 palyr.readFromLayer(crap)
 palyr.enabled = True
 palyr.fieldName = 'Name'
 palyr.placement= QgsPalLayerSettings.OverPoint
 palyr.setDataDefinedProperty(QgsPalLayerSettings.Size,True,True,'8','')
 palyr.writeToLayer(crap)

Best Answer

Place the expression in palyr.fieldName, then set palyr.isExpression to True:

palyr = QgsPalLayerSettings()
palyr.readFromLayer(crap)
palyr.enabled = True
palyr.fieldName = "concat('N:' ,round(Value,0),'\\n','Yield:' ,round(Value 2,0))"
palyr.isExpression = True
palyr.placement= QgsPalLayerSettings.OverPoint

Related Solutions

[GIS] Selecting features using expression with ‘maximum’ aggregates function in QGIS python console

Not sure if it's a bug or something else but a possible workaround could be to determine the maximum value using the QgsVectorLayer::maximumValue() method and putting this into the QgsExpression:

layer = iface.activeLayer()
field_name = 'ELEV'
field = layer.fieldNameIndex(field_name)
max_value = str(layer.maximumValue(field))
expr = QgsExpression(field_name + ' = ' + max_value)
selection = layer.getFeatures(QgsFeatureRequest(expr))
ids = [s.id() for s in selection]
layer.setSelectedFeatures(ids)

PyQGIS Field Calculation – How to Use QgsExpression to Categorize Field Values into a New Field

In QGIS expressions, double quotes are strictly for field names, single quotes are string literals, so I guess that in your Python attempt, the expression evaluation is looking for fields called "quad-1", "quad-2" etc. Therefore, you should remove the double quotes from your string values.

I have found that parsing QGIS expressions into Python syntax can be tricky. I have also found in the past that wrapping the entire expression in triple double quotes often works. I tested this with your expression and it worked for me.

Modified code:

pv = layer.dataProvider()
pv.addAttributes([QgsField('az_quad', QVariant.String)])

layer.updateFields()

e = QgsExpression("""CASE
                WHEN (0.000 <= "azimuth" AND "azimuth" <= 90.000 )THEN 'quad-1' 
                WHEN (90.000 < "azimuth" AND "azimuth"  <= 180.000 )THEN 'quad-2'
                WHEN (180.000 < "azimuth" AND "azimuth" <= 270.000 )THEN 'quad-3'
                WHEN (270.000 < "azimuth" AND "azimuth" <= 360.000 )THEN 'quad-4'
                END""")

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(layer))

with edit(layer):
    for f in layer.getFeatures():
        context.setFeature(f)
        f['az_quad'] = e.evaluate(context)
        layer.updateFeature(f)

Example result:

I played around some more and this also worked (no quotes around field names and single quotes around strings; whole expression wrapped in double quotes).

e = QgsExpression("CASE WHEN 0.000 <= azimuth AND azimuth <= 90.000 THEN 'quad-1' WHEN 90.000 < azimuth AND azimuth  <= 180.000 THEN 'quad-2' WHEN 180.000 < azimuth AND azimuth <= 270.000 THEN 'quad-3' WHEN 270.000 < azimuth AND azimuth <= 360.000 THEN 'quad-4' END")

Another option is to calculate and update the new values entirely with PyQGIS. An example of that (which produced the identical result) looks like this:

pv = layer.dataProvider()
pv.addAttributes([QgsField('az_quad', QVariant.String)])

layer.updateFields()

# retrieve index of 'az_quad' field
fld_idx = layer.fields().lookupField('az_quad')

# a dictionary which will be populated with feature id keys and
# value dictionaries composed of field index and attribute value
att_map = {}

# loop through features and populate attribute map with values calculated
# from value in 'azimuth' field
for f in layer.getFeatures():
    az = f['azimuth']
    if az == NULL:
        continue
    if 0.000 <= az <= 90.000:
        att_map[f.id()]={fld_idx: 'quad-1'}
    if 90.000 < az <= 180.000:
        att_map[f.id()]={fld_idx: 'quad-2'}
    if 180.000 < az <= 270.000:
        att_map[f.id()]={fld_idx: 'quad-3'}
    if 270.000 < az <= 360.000:
        att_map[f.id()]={fld_idx: 'quad-4'}

# finally, write attributes to layer provider
pv.changeAttributeValues(att_map)

Best Answer

Related Solutions

[GIS] Selecting features using expression with ‘maximum’ aggregates function in QGIS python console

PyQGIS Field Calculation – How to Use QgsExpression to Categorize Field Values into a New Field

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