input: Write a function to generate a random string of letters and digits.
  assert len(randomString()) == 8  # assuming the function is supposed to generate an 8-character string
  assert isinstance(randomString(), str)
  assert all(c.isalnum() for c in randomString())
output: import random
  import string
  
  def randomString(length=8):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))