This paper introduces ComposAR, a tool to allow a wide audience to author AR and MR applications. It is unique in that it supports both visual programming and interpretive scripting, and an immediate mode for runtime testing. ComposAR is written in Python which means the user interface and runtime behavior can be easily customized and third-party modules can be incorporated into the authoring environment. We describe the design philosophy and the resulting user interface, lessons learned and directions for future research.