diff --git a/requests/sessions.py b/requests/sessions.py
index f2b60ec8..e5fdf679 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -176,6 +176,10 @@ class Session(SessionRedirectMixin):
       200
     """
 
+    __attrs__ = [
+        'headers', 'cookies', 'auth', 'timeout', 'proxies', 'hooks',   
+        'params', 'verify', 'cert', 'prefetch']
+
     def __init__(self):
 
         #: A case-insensitive dictionary of headers to be sent on each