input: Write a python function to generate a random string of a given length using ascii letters, digits, and punctuation.
  assert len(random_string(20)) == 20
  assert len(random_string(15)) == 15
  assert len(random_string(0)) == 0
output: import random
  import string
  
  def random_string(length):
      characters = string.ascii_letters + string.digits + string.punctuation
      return ''.join(random.choice(characters) for _ in range(length))