The Python slice notation makes it very easy to access sub-lists from an existing list.

Using the square bracket notation [ ] at the end of any list enables one to quickly create a subset. The square brackets expect a minimum of 2 parameters and an optional third parameter. Can you guess what the 2 parameters will be? Yeah sure a start and stop parameter. Like anything you want to slice in life you need to know from where to where and then an increment / interval if not the default of 1

list[start:stop:step]
# defaults
# start = 0
# stop = len(list) - 1
# step = 1

Just remember how list index works, index always starts at 0 which usually makes the last index the length or count of the list minus 1

                +---+---+---+---+---+---+
                | P | y | t | h | o | n |
                +---+---+---+---+---+---+
Slice position: 0   1   2   3   4   5   6
Index position:   0   1   2   3   4   5

Get a single item from a list

How to get a single item from a list. I will not consider it to be a slice as it does not follow the slice convention, but it is technically still a small slice.

lst = [1,2,3,4,5,6,7,8,9,10]

We will be using the above list variable throughout

lst[0] # first item
1
lst[-1] # last item - same as lst[len(lst)-1]
10

Basic Slices

Specific subset with a known start and end point We would like the subset from the second items to the 5th item in this numeric list

lst[1:5]
[2, 3, 4, 5]

The second item will have an index of 1 and the 5th item will be at index 4 which will be accessible by using index 5 as python will slice to the end index minus 1, therefore specifying index as 5 will only return up to index 4.

Subset from the beginning of the list to a specified end point Let’s subset the first 6 item of the list

lst[:6]
[1, 2, 3, 4, 5, 6]

If the first parameter is left blank, Python will automatically assume that it should start slicing from the beginning of the list.

Subset the end of the list from a specified start point Let’s subset the last 6 items of the list

lst[4:]
[5, 6, 7, 8, 9, 10]

An empty stop parameter will lead to the slicing occurring to the end of the list by default.

Subset the entire list / copy the entire list

copy_lst = lst[:]
print(copy_lst)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Negative Index Slices

Using negative slices provides more flexibility

Subset an entire list except the last 2 items

lst[:-2]
[1, 2, 3, 4, 5, 6, 7, 8]

Subset only the last 2 items of the list

lst[-2:]
[9, 10]

Steps or Intervals within a slice

Subset the entire list / copy the entire list in reverse

lst[::-1]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Subset the first 2 items in reverse

lst[1::-1]
[2, 1]

Subset the last 2 items in reverse

lst[:-3:-1]
[10, 9]

Subset everything except the last two items in reverse order

lst[-3::-1]
[8, 7, 6, 5, 4, 3, 2, 1]