create a main driver than repeatedly calls collatz() and determines the length of the returned list.

def collatz_length(num):
  length = 1
  while num != 1:
    if num % 2 == 0:
      num = num // 2
      length += 1
    else:
      num = 3 * num + 1
      length += 1
  return length

def main():
  max_length = 1
  max_length_num = 1
  for i in range(1, 1000000):
    if collatz_length(i) > max_length:
      max_length = collatz_length(i)
      max_length_num = i
  return max_length_num

if __name__ == "__main__":
  print(main())