pyqgis standalone script fails to execute clip algorithm

Question

A previous question has established how to invoke QGIS's clip algorithm at the python console. I have done so and demonstrated that the method works satisfactorily on my data.

I would instead like to use this within a standalone script. To do so I've followed this to create the script below.

When I do so I get the following error message:

Unable to execute algorithm Wrong parameter value: ./path/to/highways.shp

Can anyone outline what is going on here?

import sys
import qgis.core
import qgis.core as qgc
import PyQt4.QtGui

# Initialise the application
app = PyQt4.QtGui.QApplication([])
qgc.QgsApplication.setPrefixPath('/usr/bin/qgis')
qgc.QgsApplication.initQgis()

# Prepare processing framework
sys.path.append('/usr/share/qgis/python/plugins')
from processing.core.Processing import Processing
Processing.initialize()
import processing.tools

vlayer = qgc.QgsVectorLayer("./path/to/highways.shp", "highways", "ogr")

clayer = qgc.QgsVectorLayer("./path/to/boundaries.shp", "boundaries", "ogr")

processing.tools.general.runalg("qgis:clip", vlayer, clayer, "clip_from_script.shp")

qgc.QgsApplication.exitQgis()
PyQt4.QtGui.QApplication.exit()

Answer 1

Looking at the code that complains, it appears it is not getting an object (type) it expected. Make sure your layer loading works first —for starters I'd try absolute paths if you're not sure the working dir is where you expect it to be.