input: Write a function to generate a random string of a given length. The function should use lowercase letters from the English alphabet. If no length is provided, the function should generate a string of length 6 by default.
  assert random_string(6) != random_string(6)
  assert len(random_string(5)) == 5
  assert random_string() != random_string()
output: import random
  import string
  
  def random_string(length=6):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))