Performing basic labelling with PyQGIS

Question

I'm trying to display labels using PyQGIS.

In this case, I create the layers manually, so I guess you need to add a "Label" attribute to the feature as well.

Therefore, I've written a little code whereof I think "should" work by following @Kezah's answer for QGIS 3.0: Question

The QGIS version I'm using is QGIS 3.22.

The problem is that no label is being shown.

Does perhaps somebody know a basic example of how to approach this correctly?

import sys
from qgis.gui import *
from qgis.core import *
from qgis.PyQt.QtWidgets import QMainWindow, QApplication
from qgis.PyQt.Qt import QVariant

class TestClass(QMainWindow):

    def __init__(self):
        QMainWindow.__init__(self)

        self.canvas = QgsMapCanvas()
        self.project = QgsProject.instance()
        self.setCentralWidget(self.canvas)
        self.toolPan = QgsMapToolPan(self.canvas)

        self.pointl = QgsVectorLayer("Polygon?crs=epsg:4326", "poly", "memory")
        self.pointl.startEditing()
        geom = QgsGeometry()
        geom = QgsGeometry.fromWkt("POLYGON ((4.3003272 51.3235323, 4.3003272 51.3235367, 4.3003415 51.3235456, 4.3003407 51.3235478, 4.3003637 51.3235627, 4.3003736 51.3235524, 4.3003709 51.3235438, 4.3003673 51.3235367, 4.3003644 51.3235313, 4.3003579 51.3235134, 4.3003272 51.3235323))")
        feat = QgsFeature()
        feat.setGeometry(geom)
        self.pointl.dataProvider().addFeature(feat)
        self.pointl.dataProvider().addAttributes([QgsField("label", QVariant.String)])
        self.pointl.updateFields()
        for feat in self.pointl.getFeatures():
            feat.setAttribute("label", "Test Label")
        label_settings = QgsPalLayerSettings()
        label_settings.drawLabels = True
        label_settings.fieldName = 'label'
        self.pointl.setLabelsEnabled(True)
        self.pointl.setLabeling(QgsVectorLayerSimpleLabeling(label_settings))
        self.pointl.commitChanges()
        
        self.project.addMapLayer(self.pointl)
        self.canvas.setLayers([self.pointl])
        self.canvas.setExtent(self.pointl.extent())
        self.pan()
    
    def pan(self):
        self.canvas.setMapTool(self.toolPan)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    test = TestClass()
    test.show()
    sys.exit(app.exec_())

Answer 1

There is no problem with @Kezah's answer. I've edited the script and it works. Please check it. Avoid using startEditing() and dataProvider().addFeatures() together. Use either one. If you construct a large dataset, using dataProvider().addFeatures() is usually faster.

import sys
from qgis.gui import *
from qgis.core import *
from qgis.PyQt.QtWidgets import QMainWindow, QApplication
from qgis.PyQt.Qt import QVariant


class TestClass(QMainWindow):
    def __init__(self):
        QMainWindow.__init__(self)

        self.canvas = QgsMapCanvas()
        self.project = QgsProject.instance()
        self.setCentralWidget(self.canvas)
        self.toolPan = QgsMapToolPan(self.canvas)
        
        ###### Edited Area
        self.pointl = QgsVectorLayer("Polygon?crs=epsg:4326", "poly", "memory")
        self.pointl.dataProvider().addAttributes([QgsField("label", QVariant.String)])
        self.pointl.updateFields()

        geom = QgsGeometry()
        geom = QgsGeometry.fromWkt("POLYGON ((4.3003272 51.3235323, 4.3003272 51.3235367, 4.3003415 51.3235456, 4.3003407 51.3235478, 4.3003637 51.3235627, 4.3003736 51.3235524, 4.3003709 51.3235438, 4.3003673 51.3235367, 4.3003644 51.3235313, 4.3003579 51.3235134, 4.3003272 51.3235323))")

        feat = QgsFeature(self.pointl.fields())
        feat.setGeometry(geom)
        feat["label"] = "Test Label"
        self.pointl.dataProvider().addFeatures([feat])
        ######

        label_settings = QgsPalLayerSettings()
        label_settings.drawLabels = True
        label_settings.fieldName = 'label'
        self.pointl.setLabelsEnabled(True)
        self.pointl.setLabeling(
            QgsVectorLayerSimpleLabeling(label_settings))

        self.project.addMapLayer(self.pointl)
        self.canvas.setLayers([self.pointl])
        self.canvas.setExtent(self.pointl.extent())
        self.pan()

    def pan(self):
        self.canvas.setMapTool(self.toolPan)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    test = TestClass()
    test.show()
    sys.exit(app.exec_())