Slicing is a form of extended indexing which allows us to refer to portions of a list. To use it we pass the start and stop indices of a half-open range, separated by a colon, as the square-brackets index argument. Here's how:

>>> s = [3, 186, 4431, 74400, 1048443]
>>> s[1:3]
[186, 4431]

See how the second index is one beyond the end of the returned range. The slice [1:4] . Slicing extracts part of a list. The slice range is half-open, so the value at the stop index is not included:

This facility can be combined with negative indices. For example, to take all elements except the first and last:

>>> s[1:-1]
[186, 4431, 74400]

The slice s[1:-1] is useful for excluding the first and last elements of a list as shown in the following image:

Both the start and stop indices are optional. To slice all elements from the third to the end of the list:

>>> s[3:]
[74400, 1048443]

The slice s[3:] retains all the elements from the fourth, up to and including the last element, as shown in the following diagram 

To slice all elements from the beginning up to, but not including, the third:

>>> s[:3]
[3, 186, 4431]

The slice s[:3] retains all elements from the beginning of the list up to, but not including, the fourth element as shown in the following diagram:

Notice that these two lists are complementary, and together form the whole list, demonstrating the convenience of the half-open range convention.

Since both start and stop slice indices are optional, it's entirely possible to omit both and retrieve all of the elements:

>>> s[:]
[3, 186, 4431, 74400, 1048443]

This is a called a full slice, and it's an important technique in Python.

The slice s[:] is the full-slice and contains all of the elements from the list. It's an important idiom for copying lists: