How do you use credentials saved by the browser in auto login script in python 2.7?

This is because selenium doesn't use your default browser instance, it opens a different instance with a temporary (empty) profile.

If you would like it to load a default profile you need to instruct it to do so.

Here's a chrome example:

from selenium import webdriver
from selenium.webdriver.chrome.options import Options

options = webdriver.ChromeOptions() 
options.add_argument("user-data-dir=C:\\Path") #Path to your chrome profile
w = webdriver.Chrome(executable_path="C:\\Users\\chromedriver.exe", chrome_options=options)

And here's a firefox example:

from selenium import webdriver
from selenium.webdriver.firefox.webdriver import FirefoxProfile

profile = FirefoxProfile("C:\\Path\\to\\profile")
driver = webdriver.Firefox(profile)

Here we go, just dug up a link to this in the (unofficial) documentation. Firefox Profile and the Chrome driver info is right underneath it.