--- a/python/events.py	Tue Mar 24 14:17:12 2015 +0100
+++ b/python/events.py	Tue Mar 24 18:11:28 2015 +0100
@@ -91,13 +91,13 @@
 
     def __init__(self, num = None, timeInterval = None, roaduserNum1 = None, roaduserNum2 = None, roadUser1 = None, roadUser2 = None, categoryNum = None):
         moving.STObject.__init__(self, num, timeInterval)
-        if timeInterval == None and roadUser1 != None and roadUser2 != None:
+        if timeInterval is None and roadUser1 is not None and roadUser2 is not None:
             self.timeInterval = roadUser1.commonTimeInterval(roadUser2)
         self.roadUser1 = roadUser1
         self.roadUser2 = roadUser2
-        if roaduserNum1 != None and roaduserNum2 != None:
+        if roaduserNum1 is not None and roaduserNum2 is not None:
             self.roadUserNumbers = set([roaduserNum1, roaduserNum2])
-        elif roadUser1 != None and roadUser2 != None:
+        elif roadUser1 is not None and roadUser2 is not None:
             self.roadUserNumbers = set(roadUser1.getNum(), roadUser2.getNum())
         else:
             self.roadUserNumbers = None
@@ -116,9 +116,9 @@
             self.roadUser2 = objects[nums[1]]
 
         i = 0
-        while i < len(objects) and self.roadUser2 == None:
+        while i < len(objects) and self.roadUser2 is None:
             if objects[i].getNum() in nums:
-                if self.roadUser1 == None:
+                if self.roadUser1 is None:
                     self.roadUser1 = objects[i]
                 else:
                     self.roadUser2 = objects[i]
@@ -140,7 +140,7 @@
         self.roadUser2.plotOnWorldImage(nPixelsPerUnitDistance, options, withOrigin, timeStep, **kwargs)
 
     def play(self, videoFilename, homography = None, undistort = False, intrinsicCameraMatrix = None, distortionCoefficients = None, undistortedImageMultiplication = 1.):
-        if self.roadUser1 != None and self.roadUser2 != None:
+        if self.roadUser1 is not None and self.roadUser2 is not None:
             cvutils.displayTrajectories(videoFilename, [self.roadUser1, self.roadUser2], homography = homography, firstFrameNum = self.getFirstInstant(), lastFrameNumArg = self.getLastInstant(), undistort = undistort, intrinsicCameraMatrix = intrinsicCameraMatrix, distortionCoefficients = distortionCoefficients, undistortedImageMultiplication = undistortedImageMultiplication)
         else:
             print('Please set the interaction road user attributes roadUser1 and roadUser1 through the method setRoadUsers')
@@ -192,7 +192,7 @@
             route1= getRoute(self.roadUser1,prototypes,objects,noiseEntryNums,noiseExitNums,useDestination)
             route2= getRoute(self.roadUser2,prototypes,objects,noiseEntryNums,noiseExitNums,useDestination)
 
-        if timeInterval != None:
+        if timeInterval is not None:
             commonTimeInterval = timeInterval
         else:
             commonTimeInterval = self.timeInterval
@@ -221,13 +221,13 @@
 
 def createInteractions(objects, _others = None):
     '''Create all interactions of two co-existing road users'''
-    if _others != None:
+    if _others is not None:
         others = _others
 
     interactions = []
     num = 0
     for i in xrange(len(objects)):
-        if _others == None:
+        if _others is None:
             others = objects[:i]
         for j in xrange(len(others)):
             commonTimeInterval = objects[i].commonTimeInterval(others[j])