how to switch driver in python selenium

1import unittest
2from selenium import webdriver
3from selenium.webdriver.common.keys import Keys
4from selenium.webdriver.common.by import By
5
6class GoogleOrgSearch(unittest.TestCase):
7
8     def setUp(self):
9         self.driver = webdriver.Firefox()
10
11    def test_google_search_page(self):
12         driver = self.driver
13         driver.get("http://www.cdot.in")
14         window_before = driver.window_handles[0]
15         print window_before
16         driver.find_element_by_xpath("//a[@href='http://www.cdot.in/home.htm']").click()
17         window_after = driver.window_handles[1]
18         driver.switch_to_window(window_after)
19         print window_after
20         driver.find_element_by_link_text("ATM").click()
21         driver.switch_to_window(window_before)
22
23
24    def tearDown(self):
25    self.driver.close()
26
27if __name__ == "__main__":
28unittest.main()