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']

http://effbot.org/zone/default-values.htm