How slicing in Python works

fixit_fiona

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.

Originally asked by Simon on Stack Exchange (stackoverflow) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

The syntax is: a[start:stop] # items start through stop-1 a[start:] # items start through the rest of the array a[:stop] # items from the beginning through stop-1 a[:] # a copy of the whole array There is also the step value, which can be used with any of the above: a[start:stop:step] # start through not past stop, by step The key point to remember is that the :stop value represents the first value that is not in the selected slice. So, the difference between stop and start is the number of elements selected (if step is 1, the default). The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning. So: a[-1] # last...

Full answer by Greg Hewgill: https://stackoverflow.stackexchange.com/a/509295 (CC BY-SA 4.0).

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