我在python中有以下代码

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import Select
from selenium.common.exceptions import NoSuchElementException
from unittestzero import Assert
from selenium.webdriver.support.ui import WebDriverWait
from selenium.common.exceptions import ElementNotVisibleException
import unittest, time, re

class HomePageTest(unittest.TestCase):
    expected_title="  some title here "
    def setUp(self):
        self.driver = webdriver.Firefox()
        self.driver.implicitly_wait(30)
        self.base_url = "https://somewebsite.com"
        self.verificationErrors = []

    def test_home_page(self):
        driver=self.driver
        driver.get(self.base_url)
        print "test some things here"




    def test_whatever(self):
        print "test some more things here"

    def tearDown(self):
        self.driver.quit()


if __name__ == "__main__":
    unittest.main()

我的问题是在test_home_page函数之后，firefox实例关闭并为下一个test_whatever函数再次打开。我该怎么做，以便所有测试用例都从同一firefox实例执行。