  Software that processes real-world data or that models a physical system must
have some way of managing units. While simple approaches like the understood
convention that all data are in a unit system (such as the MKS SI unit system)
do work in practice, they are fraught with possible sources of error both by
developers and users of the software. In this paper we present unyt, a Python
library based on NumPy and SymPy for handling data that has units. It is
designed both to aid quick interactive calculations and to be tightly
integrated into a larger Python application or library. We compare unyt with
two other Python libraries for handling units, Pint and astropy.units, and find
that unyt is faster, has higher test coverage, and has fewer lines of code.
