input: Write a python function to generate a random password of a given length using letters, digits, and punctuation.
  assert len(generate_password(8)) == 8
  assert len(generate_password(12)) == 12
  assert len(generate_password(16)) == 16
output: import random
  import string
  
  def generate_password(length):
      characters = string.ascii_letters + string.digits + string.punctuation
      password = ''.join(random.choice(characters) for i in range(length))
      return password