Thanks for the insights. I solved the problem as follows: I created a new class method called cleanUp, which resets NStocks to an empty list and N1 to 0. Works like a charm - it's the first time I've used a class method, and I immediately see its utility. Thanks again
class Stock(object): NStocks = [] #Class variables N1 = 0 @classmethod def cleanUp(cls): Stocks.NStocks = [] Stocks.N1 = 0 def simulation(N, par1, par2, idList, returnHistoryDir): Stock.cleanUp() results = ...... print results. -- http://mail.python.org/mailman/listinfo/python-list