PyQGIS Custom Application – Fix No GUI Appearing Issue

Question

I'm following the following instructions on getting Python to work with QGIS, outside the QGIS GUI itself: Using PyQGIS in custom applications.

But for some reason the GUI is not appearing, yet the layer seems to be loading fine and it also returns the correct feature count. What am I doing wrong?

from qgis.core import *

qgs = QgsApplication([], True)

qgs.initQgis()

vlayer = QgsVectorLayer("test.gpkg")
if not vlayer.isValid():
    raise Exception("Layer failed to load!")


project = QgsProject.instance()
project.addMapLayer(vlayer)

print(vlayer.featureCount());

# Write your code here to load some layers, use processing
# algorithms, etc.

qgs.exitQgis()

Answer 1

Any GUI doesn't appear when using PyQGIS standalone application. You have to construct it.

Here is an example:

from qgis.core import *
from qgis.gui import *
from qgis.PyQt.QtWidgets import *

### GUI Construction ###
class MapViewer(QMainWindow):
    def __init__(self):
        QMainWindow.__init__(self, None)

        self._canvas = QgsMapCanvas()
        self._root = QgsProject.instance().layerTreeRoot()

        self.bridge = QgsLayerTreeMapCanvasBridge(self._root, self._canvas)
        self.model = QgsLayerTreeModel(self._root)
        self.model.setFlag(QgsLayerTreeModel.ShowLegend)
        self.layer_treeview = QgsLayerTreeView()
        self.layer_treeview.setModel(self.model)

        self.layer_tree_dock = QDockWidget("Layers")
        self.layer_tree_dock.setObjectName("layers")
        self.layer_tree_dock.setFeatures(QDockWidget.NoDockWidgetFeatures)
        self.layer_tree_dock.setWidget(self.layer_treeview)

        self.splitter = QSplitter()
        self.splitter.addWidget(self.layer_tree_dock)
        self.splitter.addWidget(self._canvas)
        self.splitter.setCollapsible(0, False)
        self.splitter.setStretchFactor(1, 1)

        self.layout = QHBoxLayout()
        self.layout.addWidget(self.splitter)
        self.contents = QWidget()
        self.contents.setLayout(self.layout)
        self.setCentralWidget(self.contents)

        self.load_layers()

    def load_layers(self):
        # Adding a sample layer
        layer = QgsVectorLayer("test.gpkg", 'Layer1', 'ogr') # CHANGE PATH
        QgsProject.instance().addMapLayer(layer)
        self._canvas.setExtent(layer.extent())
        self._canvas.setLayers([layer])
########################
        
qgs = QgsApplication([], True)
qgs.initQgis()

### Call the GUI built by you ###
main_window = MapViewer()
main_window.show()

qgs.exec_()
qgs.exitQgis()