Multi-line labeling of a shapefile layer with pyQGis

I want to label a shp layer on two lines with pyQGis. I managed to do it with only one field, but with two or more fields (using concat) it does not work anymore.

Here is the code I wrote:

def labelLine (lyr, fieldNom1, fieldNom2):
  # This function shows the labels of 2 fields "fieldName1" and "fieldName2" for a Line layer lyr
  label = QgsPalLayerSettings ()
  label.readFromLayer (lyr)
  label.enabled = True

  # we concatenate the two fields
  label.fieldName = "concat ('BASE:', fieldName1, '\\ n', 'L =', fieldName2, 'm')"
  label.placement = QgsPalLayerSettings.Line
  label.bufferDraw = True
  label.bufferSize = 1
  label.setDataDefinedProperty (QgsPalLayerSettings.S ize, True, True, '8', '')
  label.writeToLayer (lyr)

Answer

Perhaps try something like the following expression. We can use the to_string function from the Field Calculator to convert any value from a field to a string (as you can't concatenate strings and values):

'''concat('BASE: ' + to_string("''' + fieldName1 + '''"),'\nL = ' + to_string("''' + fieldName2 + '''") + 'm')'''

---

So your code could look like:

def labelLine(lyr, fieldName1, fieldName2):
    label = QgsPalLayerSettings()
    label.readFromLayer(lyr)
    label.enabled = True
    label.fieldName = '''concat('BASE: ' + to_string("''' + fieldName1 + '''"),'\nL = ' + to_string("''' + fieldName2 + '''") + 'm')'''
    label.placement = QgsPalLayerSettings.Line
    label.bufferDraw = True

    label.bufferSize = 1
    label.setDataDefinedProperty(QgsPalLayerSettings.Size, True, True, '8', '')
    label.writeToLayer(lyr)
    lyr.setCustomProperty("labeling/drawLabels", True)
    lyr.triggerRepaint()

lyr = iface.activeLayer()
labelLine(lyr, 'fieldName1', 'fieldName2')

Example: