input: Write a function to generate a random password consisting of 8 characters. The password should include letters and digits. If no input is provided, the function should generate and return a random string with the specified properties.
  assert generate_password()== some_expected_password
  assert len(generate_password())==8
  assert any(c.isdigit() for c in generate_password())==True
  assert any(c.isalpha() for c in generate_password())==True
output: import random
  import string
  
  def generate_password():
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(8))