input: Write a function to generate a random string of a specified length. The function should return the generated string.
  
  def random_string(length):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert len(random_string(6))==6
  assert len(random_string(8))==8
  assert len(random_string(10))==10
  assert random_string(0)==''
  assert random_string(1).islower()==True
output: import random
  import string
  
  def random_string(length):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))