Don't Panic — Python Fundamentals

Python is a general-purpose language for making a sequence of statements and expressions change the state of objects. The useful mental picture is not a row of labelled boxes. A name refers to an object, and more than one name can refer to the same one. This explains the famous moment when a list changes somewhere else in the program. The list did not develop ambitions; another reference reached it.

The next surviving idea is mutation versus rebinding. Appending to a list changes one existing object, so every name that refers to it sees the change. Assigning a new list changes only that name's binding. Collections make those choices visible: lists keep ordered mutable items, tuples keep fixed groupings, dictionaries map keys to values, and sets represent unique members. Pick the one whose invariant says what the program is trying to preserve.

Functions keep that state from wandering around the building. A function takes parameters, does local work, and returns a value or produces an explicit side effect. Exceptions mark the route where normal execution cannot continue. Catch the narrow failure you can handle, then let unrelated errors keep their useful context. A context manager performs the equally unglamorous but vital job of pairing setup with cleanup. Files, locks, and temporary resources all appreciate being left in a known state.

Projects add modules, packages, tests, and a virtual environment, which isolates installed distributions for one project. The language includes a large standard library, but third-party packages arrive separately. An import package and an installable distribution can even have different names, because the universe enjoys a small paperwork puzzle.

For the full map, read the Intro first; it connects objects, control flow, functions, errors, classes, and iteration. Use Slides when the relationships need a compact view. Keep the Cheatsheet nearby while writing code, especially for equality, scope, exceptions, and environment boundaries. The practice reference then turns those concepts into a repeatable way to inspect state, and the exercise asks them to cooperate in one small report.