input: Write a python function to generate a random string of fixed length using predefined characters.
  assert len(generate_random_string()) == 5
  assert all(c in 'abcdefghijklmnopqrstuvwxyz' for c in generate_random_string())
  assert isinstance(generate_random_string(), str)
output: import random
  import string
  
  def generate_random_string(length=5):
      characters = string.ascii_lowercase
      return ''.join(random.choice(characters) for _ in range(length))