  An accurate modeling of neutrino flux attenuation and the distribution of
leptons they produce in transit through the Earth is an essential component to
determine neutrino flux sensitivities of underground, sub-orbital and
space-based detectors. Through neutrino oscillations over cosmic distances,
astrophysical neutrino sources are expected to produce nearly equal fluxes of
electron, muon and tau neutrinos. Of particular interest are tau neutrinos that
interact in the Earth at modest slant depths to produce $\tau$-leptons. Some
$\tau$-leptons emerge from the Earth and decay in the atmosphere to produce
extensive air showers. Future balloon-borne and satellite-based optical
Cherenkov neutrino telescopes will be sensitive to upward air showers from tau
neutrino induced $\tau$-lepton decays. We present nuPyProp, a python code that
is part of the nuSpaceSim package. nuPyProp generates look-up tables for exit
probabilities and energy distributions for $\nu_\tau\to \tau$ and $\nu_\mu\to
\mu$ propagation in the Earth. This flexible code runs with either stochastic
or continuous electromagnetic energy losses for the lepton transit through the
Earth. Current neutrino cross section models and energy loss models are
included along with templates for user input of other models. Results from
nuPyProp are compared with other recent simulation packages for neutrino and
charged lepton propagation. Sources of modeling uncertainties are described and
quantified.
