Python confusion: strange persistence of attributes

class App:
    def __init__(self):
        self.Target = None
    def do_thing(self):
        self.Target = TargetSelector()
        return

class SideList:
    def __init__(self, labels=[]):
        self.labels = labels

class TargetSelector:
    def __init__(self):
        self.SideList = None
        self.SideList = SideList()
        print "Reset should happen here"
        self.AddSide("left")
        self.AddSide("right")
    def AddSide(self, name):
        self.SideList.labels.append(name)

MyApp = App()
MyApp.do_thing()
print "Sidelist has", MyApp.Target.SideList.labels
MyApp.do_thing()
print "Sidelist has", MyApp.Target.SideList.labels