On Jul 15, 2:59 pm, iu2 <[EMAIL PROTECTED]> wrote: > Hi, > > I wrote this wrong recursive function that flattens a list: > > def flatten(lst, acc=[]): > #print 'acc =', acc, 'lst =', lst > if type(lst) != list: > acc.append(lst) > else: > for item in lst: > flatten(item) > return acc > > a = [1, 2, [3, 4, 5], [6, [7, 8, [9, 10], 11], 12], 13, 14] > b = flatten(a) > print b > > I was amazed to realize that it flattens the list alright. Why? 'acc' > should be an empty list on each invocation of flatten, but is seems to > accumulate anyway...
When you say acc=[] in the function declaration, it binds acc to a particular list object, rather than to a concept of an empty list. Thus, all operations being performed on acc are being performed on the same list. If, after the sample code you provided, were to call c = flatten([15,16,17,[18,19]]) print c you would get back the list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Mark Sherry -- http://mail.python.org/mailman/listinfo/python-list
