To allow arbitrarily complex computations to be performed, PSI4 is built upon the Python interpreter, with modifications termed Psithon. Sec. Psithon: Structuring an Input File describes the non-standard Python associated with clean molecule, basis, and option specification in the PSI4 input file. This documentation addresses the pure Python side- what functions allow the efficient compiled code to be run, what functions post-process and interact with that output, and how the ordinary (or ambitious) user can extent PSI4’s functionality.