arrays - removing python reference, copying instead -

so have strange error in function think must have python references rather value assignment. if can see causing value of mainlist disappear grateful. - http://pastebin.com/6szcwak8

inputs: ships -> [0] hitlistlength = 1 output of first print statement [[0]] output of second print statement [[]] 

my theory because have popped countinglist has removed value mainlist not sure how round this?

def addplacement(ships,hitlistlength,start = none, mainlist = none,countinglist = none):     if start == none:         start=0     if mainlist == none:         mainlist = []     if countinglist == none:         countinglist = []     #pop ship array use on recusion level     ship = ships.pop()     #loop through each hit     x in range(start,hitlistlength):         #add rotation counting list         countinglist.append(x)         #if don't need go deeper add counting array element of main list         if len(ships) == 0:             mainlist.append(countinglist)             print "mainlist: " + str(mainlist)         else:             #otherwise recure deeper updating mainlist             mainlist += addplacement(ships,hitlistlength,(start+1),mainlist,countinglist)         #remove loops countinglist contribution next loop can take place         countinglist.pop()     #return mainlist     ships.append(ship)     print "mainlist: " + str(mainlist)     return mainlist 

you do

mainlist.append(countinglist) 

and

countinglist.pop() 

which removes element countinglist leaves [] in mainlist

because append appends reference countinglist , pop element countinglist change gets reflected in mainlist well

use copy if want store copy of countinglist in mainlist