Python - List Slices

List Slices

List slices provide a more advanced way of retrieving values from a list. Basic list slicing involves indexing a list with two colon-separated integers. This returns a new list containing all the values in the old list between the indices.
Example:

squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[2:6])
print(squares[3:8])
print(squares[0:1])

Result:>>>
[4, 9, 16, 25]
[9, 16, 25, 36, 49]
[0]
>>>

Like the arguments to range, the first index provided in a slice is included in the result, but the second isn't.

If the first number in a slice is omitted, it is taken to be the start of the list.
If the second number is omitted, it is taken to be the end.
Example:

squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[:7])
print(squares[7:])

Result:>>>
[0, 1, 4, 9, 16, 25, 36]
[49, 64, 81]
>>>
Slicing can also be done on tuples.

List slices can also have a third number, representing the step, to include only alternate values in the slice.

squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[::2])
print(squares[2:8:3])

Result:>>>
[0, 4, 16, 36, 64]
[4, 25]
>>>
[2:8:3] will include elements starting from the 2nd index up to the 8th with a step of 3.

Negative values can be used in list slicing (and normal list indexing). When negative values are used for the first and second values in a slice (or a normal index), they count from the end of the list.

squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
print(squares[1:-1])

Result:>>>
[1, 4, 9, 16, 25, 36, 49, 64]
>>>
If a negative value is used for the step, the slice is done backward.
Using [::-1] as a slice is a common and idiomatic way to reverse a list.