On Jul 15, 9:30 pm, [EMAIL PROTECTED] wrote: > 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
I still don't understand: In each recursive call to flatten, acc should be bound to a new [], shouldn't it? Why does the binding happen only on the first call to flatten? -- http://mail.python.org/mailman/listinfo/python-list
