This is mainly documented at https://github.com/qlik-oss/server-side-extension.
If you want to write python code in Qlik Sense, please follow the instructions at https://github.com/qlik-oss/server-side-extension/blob/master/examples/python/GetStarted.md and take a look at https://github.com/qlik-oss/server-side-extension/tree/master/examples/python/fullscriptsupport_pandas.
If you want to encapsulate functionality in custom SSE functions written in python, you can start with the above then continue with https://github.com/qlik-oss/server-side-extension/tree/master/examples/python.
Axisgroup has also made a series of blog posts about SSE which I think does a good job of explaining the concepts:
- Qlik Sense Server Side Extensions Part 1/3: Architecture and Environment
- Qlik Sense Server Side Extensions Part 2/3: A Simple Example Plugin
- Qlik Sense Server Side Extensions Part 3/3: Resource Optimization and SSE Limitations