  We introduce the Generalised Lensing and Shear Spectra GLaSS code which is
available for download from https://github.com/astro-informatics/GLaSS It is a
fast and flexible public code, written in Python, that computes generalized
spherical cosmic shear spectra. The commonly used tomographic and spherical
Bessel lensing spectra come as built-in run-mode options. GLaSS is integrated
into the Cosmosis modular cosmological pipeline package. We outline several
computational choices that accelerate the computation of cosmic shear power
spectra. Using GLaSS, we test whether the assumption that using the lensing and
projection kernels for a spatially-flat universe -- in a universe with a small
amount of spatial curvature -- negligibly impacts the lensing spectrum. We
refer to this assumption as The Spatially-Flat Universe Approximation, that has
been implicitly assumed in all cosmic shear studies to date. We confirm that
The Spatially-Flat Universe Approximation has a negligible impact on Stage IV
cosmic shear experiments.
