Loading Shapefile in a standalone PyQGis App

Question

I'd like to open a shapefile in a standalone PyQGis App. The App starts and runs without a problem but a get an false-Return in the isValid()-Function on the Layer and the Layer doesn't show up.

I tried this on two different shapefiles and did make sure that the filepath is correct.

Can anyone tell me is there is something wrong or missing in the code or how I can debug this problem any further?

import sys
from PyQt4 import QtGui
from qgis.core import *
from qgis.gui import *
from PyQt4.QtCore import *

app = QtGui.QApplication(sys.argv)

window = QtGui.QMainWindow()
window.setGeometry(550, 550, 500, 300)
window.setWindowTitle("Test Window")

window_frame = QtGui.QFrame(window)
window.setCentralWidget(window_frame)
frame_layout = QtGui.QGridLayout(window_frame)

canvas = QgsMapCanvas()
frame_layout.addWidget(canvas)

filepath = "/home/stefan/testlayer/test.shp"

layer = QgsVectorLayer(filepath, "testlayer", "ogr")
QgsMapLayerRegistry.instance().addMapLayer(layer)
if not layer.isValid():
    print "Layer not valid"
canvas_layer = QgsMapCanvasLayer(layer)
canvas.setLayerSet([canvas_layer])
canvas.zoomToFullExtent()

window.show()

sys.exit(app.exec_())

Answer 1

You need to adjust a couple of things:

1. Adjust your imports according to Why the order of imports matters in a standalone PyQGIS processing script?:

   import sys
   from qgis.core import *
   from qgis.gui import *
   from PyQt4.QtCore import *
   from PyQt4 import QtGui
   

   NOTE: I'm not a big fan of importing everything (using the *). Since your script uses a few classes, you could import only them. That's optional, but better.

2. Add the following 2 lines (right below app = QtGui.QApplication(sys.argv)) to initialize the QGIS application, which registers providers and the reference system database, among other things QGIS needs, even for standalone applications, see Failed to create memory layers in QGIS application on Linux for details:

   QgsApplication.setPrefixPath("/usr", True)
   QgsApplication.initQgis()
   

That's it. Now you should get your layer loaded into the standalone application.