Overview

Every program built so far has run in isolation: no data saved between runs, no way to recover gracefully from an unanticipated input, and no way to spread growing code across more than one file. This topic gives a program three ways to interact responsibly with the world beyond its own single run: modules, for organising code across multiple files; files, for saving and loading data between runs; and exceptions and assertions, for handling the inevitable moment when something does not go as planned. docs/programme-profiles/library/introduction-to-python.md names this Stage 5 "programs that touch the outside world," and closes it with the reproducibility habits (fixed seeds, documented file formats, informative failures) that every later quant-finance and statistics lesson in this vault assumes when it shows Python code that loads data or writes results.