在python中打印数组的子集

Question

I am trying to print subsets of an array in python but i am get None type is not iterable error

def printSubs(l,out):
  if (len(l) == 0):
    print(*out)
    return

  printSubs(l[1:],out)
  printSubs(l[1:],out.append(l[0]))
printSubs([15,20,12],[])

this throws None type is not iterable error but when i copy the out into a new array and then append l[0] to it and pass it to the second function call it works fine

def printSubs(l,out):
  if (len(l) == 0):
    print(*out)
    return

  printSubs(l[1:],out)
  newOut = [i for i in out]
  newOut.append(l[0])

  printSubs(l[1:],newOut)
printSubs([15,20,12],[])

this code works fine can anyone explain me why is it happening to me it seems both code does the same thing as the out array is having different states in diffrent function calls. Thanks in advance :)

Answer 1

out.append(l[0]) doesn't return anything and that's what you're passing in function. That's why this error.

Instead of -

  printSubs(l[1:],out)
  printSubs(l[1:],out.append(l[0]))

You have to do -

  printSubs(l[1:],out)
  out.append(l[0])
  printSubs(l[1:],out)