Excel add-ins in Python

You could try xlOil (disclaimer: I wrote it). The docs are here. To write a simple three input function, you would install xlOil using:

pip install xloil
xloil install

Then write:

import xloil

@xloil.func
def myfunc(x, y, z):
    return x + y * z

Put this code either:

1. In a py file in the same directory as your spreadsheet, named Book1.py (if your sheet is called Book1.xlsx)
2. In a py file on your python module path, then edit %APPDATA%\xlOil\xlOil.ini to load it as described in the docs.

Start Excel, open your spreadsheet and the function will be available as myfunc.