diff -r 0c20fc810d61 -r 5736d25bb393 plcopen/XSLTModelQuery.py
--- a/plcopen/XSLTModelQuery.py	Tue Feb 20 11:24:40 2018 +0100
+++ b/plcopen/XSLTModelQuery.py	Tue Feb 20 14:16:51 2018 +0100
@@ -14,30 +14,33 @@
 
 class XSLTModelQuery(object):
     """ a class to handle XSLT queries on project and libs """
-    def __init__(self, controller, xsltpath, ext = []):
+    def __init__(self, controller, xsltpath, ext=None):
         # arbitrary set debug to false, updated later
         self.debug = False
 
         # merge xslt extensions for library access to query specific ones
         xsltext = [
-            ("GetProject", lambda *_ignored: 
-                [controller.GetProject(self.debug)]),
-            ("GetStdLibs", lambda *_ignored: 
-                [lib for lib in StdBlckLibs.values()]),
-            ("GetExtensions", lambda *_ignored: 
-                [ctn["types"] for ctn in controller.ConfNodeTypes])
-        ] + ext
+            ("GetProject", lambda *_ignored:
+             [controller.GetProject(self.debug)]),
+            ("GetStdLibs", lambda *_ignored:
+             [lib for lib in StdBlckLibs.values()]),
+            ("GetExtensions", lambda *_ignored:
+             [ctn["types"] for ctn in controller.ConfNodeTypes])
+        ]
 
-        # parse and compile. "beremiz" arbitrary namespace for extensions 
+        if ext is not None:
+            xsltext.extend(ext)
+
+        # parse and compile. "beremiz" arbitrary namespace for extensions
         self.xslt = etree.XSLT(
             etree.parse(
                 os.path.join(ScriptDirectory, xsltpath),
                 etree.XMLParser()),
-            extensions={ ("beremiz", name):call for name, call in xsltext})
+            extensions={("beremiz", name): call for name, call in xsltext})
 
     def _process_xslt(self, root, debug, **kwargs):
         self.debug = debug
-        res = self.xslt(root,**{k:etree.XSLT.strparam(v) for k,v in kwargs.iteritems()})
+        res = self.xslt(root, **{k: etree.XSLT.strparam(v) for k, v in kwargs.iteritems()})
         # print(self.xslt.error_log)
         return res
 
@@ -60,4 +63,3 @@
     return [translate(arg[0]) if len(arg) > 0 else None
             for translate, arg in
             zip(translations, args)]
-