S4Py is a tool and library for working with Sims 4 content. Right now it's fairly limited, but it will eventually do everything from low-level inspection of Sims4 data files to compiling a high-level description of what you want your mod to do from a single user-friendly input file into a package complete with automatically-generated string tables and SimData files (and checking to make sure that the tunables that you use are valid).
Because I've been developing software for a while now, I have fairly strong opinions about how builds should work. For example, the build tools should NEVER modify a file that you as a user directly edit; in fact, the output of a build step should never depend whether the output file existed before the build step executed, much less depend on its prior contents. This means that for the low-level build tool, if you maintain your assets as loose files in a directory and then run
s4py package convert -o mymod.package sourcedir
mymod.package will contain ONLY the resources that were in
sourcedir.
Right now s4py is not going to be useful to you unless you want to do significant development work; it's in the very early stages of development. You can expect that there will be an XML-to-SimData compiler fairly soon; the assemble tool will follow fairly shortly after that. If there are any specific tools that you need, don't hesitate to let me know by filing a feature request.
However, it is already a nice tool for browsing the contents of packages and
for building complete packages from unpacked trees; see the s4py package convert command. It can also prettyprint the internals of some resources with
s4py package cat.
First, make sure that you have Python 3.3 or above installed. Then, run
python setup.py install
If you want to work on s4py, I recommend instead running
python setup.py develop
Either way, you will then have the s4py binary available for use. Run it for further instructions.