Understanding Python's slice notation

Basic syntax

Python's slice notation is used to return a list or a portion of a list. The basic syntax is as follows:

[start_at:stop_before:step]

Where start_at is the index of the first item to be returned (included), stop_before is the index of the element before which to stop (not included) and step is the stride between any two items.

All three of the arguments are optional, meaning you can omit any of them. For example:

nums = [1, 2, 3, 4, 5]

nums[1:4]     # [2, 3, 4]   (start at 0, stop before 4)
nums[2:]      # [3, 4, 5]   (start at 0, stop at end of list)
nums[:3]      # [1, 2, 3]   (start at 0, stop before 3)
nums[1:4:2]   # [2, 4]      (start at 1, stop before 4, every 2nd element)
nums[2::2]    # [3, 5]      (start at 2, stop at end of list, every 2nd element)
nums[:3:2]    # [1, 3]      (start at 0, stop before 3, every 2nd element)
nums[::2]     # [1, 3, 5]   (start at 0, stop at end of list, every 2nd element)
nums[::]      # [1, 2, 3, 4, 5] (start at 0, stop at end of list)

As you can probably tell from the examples above, the default values are start_at = 0, stop_before = len(nums), step = 1.

An idiomatic way to shallow clone a list would be using [:] (e.g. nums_clone = nums[:]).

Negative values

All three of the arguments also accept negative values. For start_at and stop_before, a negative value means counting from the end of the list instead of counting from the start. For example -1 would represent the last element, -2 the second last element etc. For example:

nums = [1, 2, 3, 4, 5]

nums[1:-2]    # [2, 3]      (start at 1, stop before 2nd to last)
nums[-3:-1]   # [3, 4]      (start at 3rd to last, stop before last)

A negative step means that the list is sliced in reverse (from end to start). This also means that start_at should be greater than stop_before and that stop_before in the context of a reverse stride is more like stop_after if you are looking at the list non-reversed. For example:

nums = [1, 2, 3, 4, 5]

nums[::-1]    # [5, 4, 3, 2, 1]   (reversed)
nums[4:1:-1]  # [5, 4, 3]   (reversed, start at 4, stop after 1)
nums[-1:1:-2] # [5, 3]      (reversed, start at last, stop after 1, every 2nd)

Empty slices

Bear in mind that slice notation is very forgiving, so you'll get an empty list if the arguments' values are out of the list's range. For example:

nums = [1, 2, 3, 4, 5]

nums[6:8]     # []
nums[:-10]    # []

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