input: Write a function to generate a random string of a specified length using lowercase letters. The function should take an integer as input representing the length of the string and return a randomly generated string of that length. If the input length is not specified, it should default to 6.
  
  def random_string(length=6):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert random_string()=='abcdef' or random_string()!='abcdef'
  assert random_string(5)=='abcde' or random_string(5)!='abcde'
  assert random_string(0)==''
  assert random_string(10)=='abcdefghij' or random_string(10)!='abcdefghij'
output: def random_string(length=6):
      import random
      import string
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))