zfit: scalable pythonic fitting

Statistical modeling is a key element for High-Energy Physics (HEP) analysis. The standard framework to perform this task is the C++ ROOT/RooFit toolkit; with Python bindings that are only loosely integrated into the scientific Python ecosystem. In this paper, zfit, a new alternative to RooFit written in pure Python, is presented. Most of all, zfit provides a well defined high level API and workflow for advanced model building and fitting together with an implementation on top of TensorFlow. It is designed to be extendable in a very simple fashion, allowing the usage of cutting-edge developments from the scientific Python ecosystem in a transparent way. Moreover, the main features of zfit are introduced, and its extension to data analysis, especially in the context of HEP experiments, is discussed.