What is the correct way to pass a list to a Python function?

Question

Python

Wrote a function to compose all placements without repetition.

def combinator(setOfNum,length,deep=-1,arrayOfComb=[],num=[]):
  if deep == -1 : deep = length
  if deep == 0:
    arrayOfComb.append(copy.copy(num))
    return
  for i in range(setOfNum):
    if num.count(i) == 0: num.append(i)
    else: continue
    combinator(setOfNum,length,deep-1)
    del num[-1]
  if deep == length:
    return copy.copy(arrayOfComb)

The problem is that every time the function is called, the new result is added to the old one. Removing before return doesn't help. It seems to me that lists should be passed differently.