I have been writing Web API endpoints for the last few weeks. Unfortunately not in Python because the team uses another language. However, I came across two very useful free tools for testing my code, both on the client side and server side.
Client Side Postman Google Extension
This is a Chrome extension so you will need to use Google Chrome. But it is simple and flexible, add headers and bodies in different formats with a few clicks.
Server Side RequestBin
It creates a private bucket to view what your request looks like on the Server side. The only complain I have is it lacks the environment variables you would get on the web server part, but I bypass that by using a Azure website running a simple site using Bottle framework, as explained here.