how can I render html into variable..

example

If I want to render html template I use this code

return http.request.render('track_orders.t_test', {})

but what about If I want this render result into a variable like:

variable =  http.request.render('track_orders.t_test', {})

I did it but the result into the variable was "<Respond 0 byte [200 OK]>"

And I want the html rendered inside the variable