input: Write a python function to generate a 16-character password using random choice of letters and digits.
  assert len(generate_password()) == 16
  assert all(c in string.ascii_letters + string.digits for c in generate_password())
output: import random
  import string
  
  def generate_password():
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(16))