QgsMapCanvas window flashes on then disappears

Question

I am trying to make a standalone app, to be able to select and download some spatial data that I keep around. I've read around and found I can do that with Qgis and Qt, so I grabbed some example code from the docs, and assigned it a shapefile to open.

And it wouldn't work, regardless of what I tried with PATH and PROJ_LIB. While I was trying to work out my issues with:

proj_create_from_database: Cannot find proj.db
proj_get_authorities_from_database: Cannot find proj.db

I finally found out, I can avoid that by using:

C:\Program Files\QGIS 3.16\bin>python-qgis-ltr.bat myApp.py

Now a window is shown for a split second, then it disappears and the program finishes, as cmd is ready for new inputs.

Here is the whole code:

import sys
sys.path.append(r"C:\Program Files\QGIS 3.16\apps\qgis-ltr\python")
import os

os.environ['QT_QPA_PLATFORM_PLUGIN_PATH'] = r'C:\Program Files\QGIS 3.16\apps\Qt5\plugins'
os.environ['PROJ_LIB '] = r'C:\Program Files\QGIS 3.16\share\proj'
os.environ['PYTHONPATH'] = r'C:\Program Files\QGIS 3.16\apps\Python37'

from qgis.PyQt.QtGui import (
    QColor,
)

from qgis.PyQt.QtCore import Qt, QRectF

from qgis.core import (
    QgsVectorLayer,
    QgsPoint,
    QgsPointXY,
    QgsProject,
    QgsGeometry,
    QgsMapRendererJob,
)

from qgis.core import *

from qgis.gui import (
    QgsMapCanvas,
    QgsVertexMarker,
    QgsMapCanvasItem,
    QgsRubberBand,
    QgsMapToolPan,
    QgsMapToolZoom,
)

from qgis.PyQt.QtWidgets import QAction, QMainWindow

class MyWnd(QMainWindow):
    def __init__(self, layer):
        QMainWindow.__init__(self)

        self.canvas = QgsMapCanvas()
        self.canvas.setCanvasColor(Qt.white)

        self.canvas.setExtent(layer.extent())
        self.canvas.setLayers([layer])

        self.setCentralWidget(self.canvas)

        self.actionZoomIn = QAction("Zoom in", self)
        self.actionZoomOut = QAction("Zoom out", self)
        self.actionPan = QAction("Pan", self)

        self.actionZoomIn.setCheckable(True)
        self.actionZoomOut.setCheckable(True)
        self.actionPan.setCheckable(True)

        self.actionZoomIn.triggered.connect(self.zoomIn)
        self.actionZoomOut.triggered.connect(self.zoomOut)
        self.actionPan.triggered.connect(self.pan)

        self.toolbar = self.addToolBar("Canvas actions")
        self.toolbar.addAction(self.actionZoomIn)
        self.toolbar.addAction(self.actionZoomOut)
        self.toolbar.addAction(self.actionPan)

        # create the map tools
        self.toolPan = QgsMapToolPan(self.canvas)
        self.toolPan.setAction(self.actionPan)
        self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in
        self.toolZoomIn.setAction(self.actionZoomIn)
        self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out
        self.toolZoomOut.setAction(self.actionZoomOut)

        self.pan()

    def zoomIn(self):
        self.canvas.setMapTool(self.toolZoomIn)

    def zoomOut(self):
        self.canvas.setMapTool(self.toolZoomOut)

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

qgs = QgsApplication([], True)

qgs.initQgis()

pathToShp = r"D:\Docs\myTestShape.shp"
inLayer = QgsVectorLayer(pathToShp, "myTestShape", "or")

w = MyWnd(inLayer)
w.show()

What do I need to do to see something displayed and pan around it?

Answer 1

You need to add at the end of your code the following

exitcode = qgs.exec()
QgsApplication.exitQgis()
sys.exit(exitcode)