docs: def generate_random_string(length=6):
      '''
      Returns a random string of a specified length.
  
      >>> len(generate_random_string(length=25))
      25
  
      Test randomness. Try N times and observe no duplicaton
      >>> N = 100
      >>> len(set(generate_random_string(10) for i in range(N))) == N
      True
      '''
      n = int(length / 2 + 1)
      x = binascii.hexlify(os.urandom(n))
      s = x[:length]
      return s.decode('utf-8')