Using a custom Shortcut, the user chooses a budgeting function (like adding a credit card purchase or logging a deposit) and enters all relevant values into the Shortcut's input prompt.
The user input is parsed into arguments (by newline), then encoded and concatenated to a Safari webhook, which launches main.py via Pythonista 3, passing in the arguments as sys.args[1:0]. Arguments are handled according to the user-selected function, each of which reads and modifies the most recent budget file in the budgets/ directory.
Pythonista doesn't currently support accessing iCloud files outside its own working directory, so the budgets/ directory sits at the root of //iCloud/Pythonista/. Working Copy syncs its local copy of this repo to //iCloud/Pythonista/budget-keeper/ so that it can access the budget files.