Opening vector layer in QGIS Python standalone application?

Question

I'm working on a project using the QGIS Python API. I seem to be having some issues with opening a vector layer from a shapefile in my standalone application. This is the Python script I'm using to test opening the layer:

# Import statements
from qgis.core import *
import os
# Instantiate the QGIS Application
GUIEnabled = True
app = QgsApplication([], GUIEnabled)
# Update prefix path
app.setPrefixPath("C:\OSGeo4W64\apps\qgis", True)
app.initQgis()
# File to be read by the application
vector_source = "absolute_path_to_file"
# Read layer from file
layer = QgsVectorLayer(vector_source, "Vector_Layer", "ogr")
# DEBUG: Test if the file path points to a valid file
print "File Path points to a file: ", os.path.isfile(vector_source)
# DEBUG: Test if the layer initialized correctly
print "Output of layer.isValid(): ", layer.isValid()

layer.isValid() returns false when I run the script. I checked to make sure the file was valid by opening it in the QGIS GUI, which worked. I also checked out the answer to Creating QGIS layers in python console vs stand-alone application, but the answer there is specific to Mac users, and I am running on Windows 8.

Answer 1

You need to escape your backslashes when you defined your paths for the QGIS application (I tend to use forward slashes).

So you should replace:

app.setPrefixPath("C:\OSGeo4W64\apps\qgis", True)

with:

app.setPrefixPath("C:/OSGeo4W64/apps/qgis", True)

or:

app.setPrefixPath(r"C:\OSGeo4W64\apps\qgis", True)

---

Example

• Benjamin: Thank you. This resolved the issue I was facing. It's strange that this wasn't a problem in the Python console in QGIS.
• Joseph: @Benjamin - Most welcome, glad it helped! You mean using single backslashes isn't a problem in the console?
• Benjamin: It wasn't when I tested the code in the console that's in the QGIS GUI
• Joseph: @Benjamin - Yeah I think QGIS is quite forgiving when it comes to little things like this and solves it automatically. Or atleast that's what I experienced several times =)