Why auth must be prepared last

requests/models.py

@@ -195,6 +195,8 @@ class Request(RequestHooksMixin):
         p.prepare_headers(self.headers)
         p.prepare_cookies(self.cookies)
         p.prepare_body(self.data, self.files)
+        # Note that prepare_auth most be last to enable authentication schemes
+        # such as OAuth to work on a fully prepared request.
         p.prepare_auth(self.auth)
 
         return p