Modules, packages, and virtual environments

One environment per project, always

Installing packages globally means every project shares one set of versions, and the first version conflict breaks something you were not working on. A virtual environment gives each project its own isolated set.

A module is just a file, and a package a directory of them. The import guard that checks whether a file is being run directly is what lets the same file work as both a script and an importable module.

Record your dependencies in a file and pin versions for anything you will run again. An unpinned environment reproduces differently every month.