I have a function (test) with a list variable APP and declared its default as an empty list [], while APP is not a global variable, if I execute the same function multiple times, each time the APP will get appended. To workaround this problem, I need to do APP.pop() inside the function or explicitly called the function with an argument (test([])). del APP or reassign APP=[] inside the function does not resolve the problem same thing happens if the function is defined as an method inside a class. Here is a little test program for testing: def test(APP=[]): if len(APP) == 0: APP.append('1abc') print "APP=", APP APP.append('2def') class test1 (object): def t1(self, abc=[]): abc.append('abc') print abc if __name__ == '__main__': print "class test" t = test1() i = 0 while i < 3: t.t1() i += 1 print "Test function::" i = 0 while i < 3: test() i += 1 Here are the output: class test ['abc'] ['abc', 'abc'] ['abc', 'abc', 'abc'] Test function:: APP= ['1abc'] APP= ['1abc', '2def'] APP= ['1abc', '2def', '2def']