The Nutshell Python SDK by Emil Stenström allows API interaction and provides JSON-RPC protocol. Resources explain how to create an API key and a username to make calls. Documentation available at https://www.nutshell.com/api/detail/class_core.html
Square has launched the Register API for Android only a couple of months after releasing the API for iOS. The API allows developers to build end to end point-of-sale solutions that utilize Square's entire payment platform, or that integrate with third party infrastructure and applications.