Using standalone pyQGIS application QgsApplication.initQgis() fails?

I'm trying to execute the following code on a windows 7 system:

import qgis.core as qc
qgis_path = "C:\Program Files\QGIS Lyon\apps\qgis"

qc.QgsApplication.setPrefixPath(qgis_path, True)
qc.QgsApplication.initQgis()
print "Successfully Initialised"

The qc.QgsApplication.initQgis() command causes python to stop working. Can anyone help with the steps I need to diagnose this problem?

Some background:

• I've tried a number of different combinations of forward slashes, escaped backslashes etc.for the path and still get the same error;


• I've been developing the code (part of a larger application) on Linux, without problems;


• A long time ago, I tested the code on Windows and it worked OK. At that time I had QGIS Dufour installed;



• I suspect it may have something to do with having the correct environment variables set, but have been unable to find a definitive guide on which and how these variables need to be set.