How to read the code

Code appears in two forms. Short excerpts show one idea in isolation, a few lines lifted out of a file so that the thing being discussed is the only thing on the page. Complete files are labelled with a line of exactly this shape, immediately above the code:

Every complete file also appears, in its finished state, in Appendix B. It is generated from the chapters themselves, so the two cannot disagree. If a file you typed does not behave as the chapter says, the appendix is the reference.

Several files carry a main function at the bottom, guarded so that it runs only when the file is the one you ran. Those files are both a module the pipeline imports and a command you can try from the terminal, and the chapter that introduces each one says what to type.

Type the code. Do not paste it.