Before you touch indexing or slicing, every tiny symbol the parent note used must be built from nothing. Below, each item gives you its plain meaning → the picture → why the topic needs it, ordered so each one leans only on the ones above it.
Look at the figure: the name a is just a tag on a string leading to a box in memory. Nothing about the box is stored inside the name a — the name only knows the address of the box. Keep this picture; the aliasing trap b = a is impossible to misread once you see two tags on one box.
Why the topic needs this: the whole "mutability" story (b = a shares, a[:] copies) is only a story about how many tags point at how many boxes.
In the figure, the forward numbers 0,1,2,3 sit under each box (how far you walked), and the backward numbers -1,-2,-3,-4 sit above (how far back from the end). This one picture is the seed of the negative-index identity a[−k]=a[n−k] the parent proves.
Why the topic needs this: "why does indexing start at 0" and "what does a[-1] mean" are both answered by this single offset picture.
Look at the figure: cutting at gap 2 and gap 5 gives you exactly boxes 2,3,4 — three boxes, and 5−2=3. That is whystop is excluded and why the slice length is simply stop−start: you are just measuring the distance between two fence-posts.
Why the topic needs this: the "why is stop excluded", the clean length formula, and the perfect join of a[:k] with a[k:] all fall out of this one gap picture.
Why the topic needs this: the parent's mixed = [1, "hi", 3.5, True] and nested = [[1,2],[3,4]] examples rely on exactly this "box holds an address" fact.
Why the topic needs this: the classic trap a = a.append(x) (which sets a = None) is only understandable once you know .appendreturns None and =re-tags.
Every arrow above points at a claim in the parent note. If any source box confuses you, re-read its section above before continuing to the parent topic.
