The _cells dictionary maps cell addresses to formulas. Looking up the cell triggers an eval on that cell formula, possibly resulting in other cells being looked up and evaluated.

The tools class variable is a dictionary of functions and constants you want to make visible to users of the spreadsheet. This doesn't make eval() totally secure, but it improves the situation somewhat.

This is meant to be a minimal example to show the basic idea. The idea scales up well. Without too much effort, I've built out a modest spreadsheet program with error trapping, limited access to globals, handling of circular references, strings, summations, a library of spreadsheet functions, the ability to save and restore (using pickle), and a TkInter GUI that looks and acts like a regular spreadsheet.