magpylib provides simple and easy to use classes for calculating magnetic fields depending only on numpy and matplotlib. The core of the library is the source class which can represent permanent magnets, current distributions or magnetic moments in a 3D space. The library provides simple ways to generate such source objects, to manipulate them geometrically, to group several sources into a Collection system and to calculate the resulting fields of such systems. The systems may also be quickly visualized with a matplotlib based 3D plot method.