Old versions of the Requests library (0.14.2 and older) put new cookies in the jar for you when you pass a CookieJar
object:
import requests
import cookielib
URL = '...whatever...'
jar = cookielib.CookieJar()
r = requests.get(URL, cookies=jar)
r = requests.get(URL, cookies=jar)
The first request to the URL fills the jar and the second request sends the cookies back to the server.
This doesn’t work starting with Requests 1.0.0, released in 2012.