Magpylib: A free Python package for magnetic field computation. This paper introduces Magpylib, a light-weight and free Python package used to compute magnetic fields of permanent magnets, currents and moments based on analytical models. The package features a user-friendly interface to create, manipulate, group and visualize such magnetic sources, and to compute the field they generate. We present an overview of structure and functionality of Magpylib, followed by a discussion of possible applications and examples of magnetic system design. Special attention is given to computational performance possibilities in this context.