  SkyPy is an open-source Python package for simulating the astrophysical sky.
It comprises a library of physical and empirical models across a range of
observables and a command-line script to run end-to-end simulations. The
library provides functions that sample realisations of sources and their
associated properties from probability distributions. Simulation pipelines are
constructed from these models using a YAML-based configuration syntax, while
task scheduling and data dependencies are handled internally and the modular
design allows users to interface with external software. SkyPy is developed and
maintained by a diverse community of domain experts with a focus on software
sustainability and interoperability. By fostering development, it provides a
framework for correlated simulations of a range of cosmological probes
including galaxy populations, large scale structure, the cosmic microwave
background, supernovae and gravitational waves.
  Version 0.4 implements functions that model various properties of galaxies
including luminosity functions, redshift distributions and optical photometry
from spectral energy distribution templates. Future releases will provide
additional modules, for example, to simulate populations of dark matter halos
and model the galaxy-halo connection, making use of existing software packages
from the astrophysics community where appropriate.
