Internal aspects of EnergyPlus are now accessible via API. The API is written in C, but a Python wrapper is also included on top of this API for convenience. More details about the API including usage details are provided in the input-output documentation which is included with the EnergyPlus install. This website documents the low-level details of the API functions.
EnergyPlus now includes a proper C API. Documentation is built using Doxygen.
The API also includes Python bindings that wrap the C API. Documentation is built using Sphinx.