Write a function that takes length as an argument and generates a random sequence of digits of length, with at least one digit 3 and one digit 7 in the sequence. The positions of 3 and 7 must be determined randomly. For example: "456379" or "033456127".

def rand_seq(length):
  import random
  seq = ""
  while True:
    if "3" in seq and "7" in seq:
      break
    seq = "".join([str(random.randint(0, 9)) for num in range(length)])
  return seq
  
rand_seq(15)