Computing quantum transport numerically is a subject that has been around for decades, and many different algorithms have been developed, such as the recursive Green's function (RGF) formalism. I will present a wave function based algorithm for computing transport properties as well as local observables (such as electron density) in non-interacting tight-binding systems. This algorithm is inherently more stable than e.g. the RGF algorithm, and is for large systems typically an order of magnitude faster. This algorithm has been implemented in the software package Kwant, and I will also show the principles behind the design the interface of such a program, and how it allows to solve even complex systems by writing a few lines of python code.