I have the following problem : I've installed all pyQGIS environment on Windows 7. I work with 64 bits version of Python 3.6 and QGIS 3.0 and PyQt5. My Windows paths are ok.

My objective is to create an empty QGS project with stand-alone python script and add layers into.

But when I try to read a shapefile (the shp is good when I open it in QGIS GUI) I get an invalid QgsVectorLayer :

import os

from qgis.core import QgsApplication, QgsProject, QgsVectorLayer
from PyQt5.QtCore import QFileInfo

QgsApplication.setPrefixPath("C:\\QGIS 3.0\\apps\\qgis", True)
qgs = QgsApplication([], False)

pathQGS = "C:\\path\\to\\project.qgs"
layerPath = "C:\\path\\to\\layer.shp"
fileInfo = QFileInfo(layerPath)
path = fileInfo.filePath()
baseName = fileInfo.baseName()

project = QgsProject.instance()
if os.path.isfile(pathQGS):
layer = QgsVectorLayer(path, baseName, 'ogr')
if not layer.isValid():
    print("Layer is not valid")


How can I solve this ? I checked many other topics without finding any answer to my problem.

Can you please help me ?

  • I just noticed you didn't describe exactly what error (messages) you are getting from running this code. Were you able to run through all lines of codes without exception? I suspect there might be some issue with the QGIS prefix path... – For Comment Jun 13 '18 at 14:59
  • Exactly. I don't get any errors but when I try for example to read columns names I get : AttributeError: 'NoneType' object has no attribute 'fieldNameByIndex' – dmjf08 Jun 13 '18 at 17:43
  • EDIT : when I change the value of QgsApplication.setPrefixPath it doesn't change anything in os.environ['QGIS_PREFIX_PATH'] ... Maybe this is the link to the issue. – dmjf08 Jun 13 '18 at 17:49
  • I think you should have called fieldNameIndex (not fieldNameByIndex). Also you should call it via layer data provider like this: layer.dataProvider().fieldNameIndex("aFieldName") – For Comment Jun 14 '18 at 15:04
  • The QgsApplication.setPrefixPath won't change the QGIS_PREFIX_PATH environ variable value. I am curious how did you install your QGIS 3 software? because that path is a little unusual. – For Comment Jun 14 '18 at 15:08

To create a QgsProject object, simply do the following:

project = QgsProject()

I tried the rest of your code and worked, it create a .qgs qgis project file, of course if your layer is not valid, it might make the .addMapLayer() call fail.

  • As I said, I opened my layer in QGIS and checked the valididty. The invalid output layer is empty. I think the problem has another reason. – dmjf08 Jun 13 '18 at 8:18
  • I added a point layer into the .qgs project and it saved and can be opened in QGIS 3.0, project and the data layer without issue. – For Comment Jun 13 '18 at 14:47

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