Setting QgsExpression for QgsPalLayerSettings in QGIS

Question

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)

Answer 1

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

Answer 2

Here's a working snippet for creating some label settings and labelling with an expression for PyQGIS 3:

# Define layer object
layer = QgsProject().instance().mapLayersByName('Name_of_your_layer')[0]

settings = QgsPalLayerSettings()
format = QgsTextFormat()
format.setFont(QFont('Arial', 8))
format.setColor(QColor('Black'))
buffer = QgsTextBufferSettings()
buffer.setEnabled(True)
buffer.setSize(0.50)
buffer.setColor(QColor('grey'))
format.setBuffer(buffer)
settings.setFormat(format)
settings.fieldName = """concat("field_1", ' ', "Field_2")"""
settings.isExpression = True
labels = QgsVectorLayerSimpleLabeling(settings)
layer.setLabelsEnabled(True)
layer.setLabeling(labels)
layer.triggerRepaint()