input: Write a python function to generate a random password of length 6 containing letters and digits.
  assert len(generate_password()) == 6
  assert any(char.isdigit() for char in generate_password())
  assert any(char.isalpha() for char in generate_password())
output: import random
  import string
  
  def generate_password():
      characters = string.ascii_letters + string.digits
      password = ''.join(random.choice(characters) for _ in range(6))
      return password