input: Write a python function to generate a random string of lowercase letters.
  assert len(random_string()) == 10
  assert random_string().islower()
  assert all(char in string.ascii_lowercase for char in random_string())
output: import random
  import string
  
  def random_string(length=10):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))