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How slicing in Python works

How does Python's slice notation work? That is: when I write code like a[x:y:z], a[:], a[::2] etc., how can I understand which elements end up in the slice?

_{See Why are slice and range upper-bound exclusive? to learn why xs[0:2] == [xs[0], xs[1]], not [..., xs[2]].}
_{See Make a new list containing every Nth item in the original list for xs[::N].}
_{See How does assignment work with list slices? to learn what xs[0:2] = ["a", "b"] does.}

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How slicing in Python works

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