diff -r 98376cf9133d Lib/http/cookiejar.py
--- a/Lib/http/cookiejar.py	Tue Jan 06 22:16:10 2015 -0800
+++ b/Lib/http/cookiejar.py	Wed Jan 07 11:21:12 2015 +0100
@@ -2016,7 +2016,8 @@
                 assert domain_specified == initial_dot
 
                 discard = False
-                if expires == "":
+                # curl and Wget set expires to 0 for session cookies.
+                if expires == "0" or expires == "":
                     expires = None
                     discard = True
 
@@ -2064,7 +2065,7 @@
                 if cookie.expires is not None:
                     expires = str(cookie.expires)
                 else:
-                    expires = ""
+                    expires = "0"
                 if cookie.value is None:
                     # cookies.txt regards 'Set-Cookie: foo' as a cookie
                     # with no name, whereas http.cookiejar regards it as a