Next / Previous / Contents / TCC Help System / NM Tech homepage

4.3. Slicing sequences

For a sequence S, and two positions B and E within that sequence, the expression S [ B : E ] produces a new sequence containing the elements of S between those two positions.

>>> L
[0, 1, 2, 3, 4, 5]
>>> L[2]
2
>>> L[4]
4
>>> L[2:4]
[2, 3]
>>> s = 'abcde'
>>> s[2]
'c'
>>> s[4]
'e'
>>> s[2:4]
'cd'

Note in the example above that the elements are selected from position 2 to position 4, which does not include the element L[4].

You may omit the starting position to slice elements from at the beginning of the sequence up to the specified position. You may omit the ending position to specify a slice that extends to the end of the sequence. You may even omit both in order to get a copy of the whole sequence.

>>> L[:4]
[0, 1, 2, 3]
>>> L[4:]
[4, 5]
>>> L[:]
[0, 1, 2, 3, 4, 5]

You can replace part of a list by using a slicing expression on the left-hand side of the “=” in an assignment statement, and providing a list of replacement elements on the right-hand side of the “=”. The elements selected by the slice are deleted and replaced by the elements from the right-hand side.

In slice assignment, it is not necessary that the number of replacement elements is the same as the number of replaced elements. In this example, the second and third elements of L are replaced by the five elements from the list on the right-hand side.

>>> L
[0, 1, 2, 3, 4, 5]
>>> L[2:4]
[2, 3]
>>> L[2:4] = [93, 94, 95, 96, 97]
>>> L
[0, 1, 93, 94, 95, 96, 97, 4, 5]

You can even delete a slice from a sequence by assigning an an empty sequence to a slice.

>>> L
[0, 1, 93, 94, 95, 96, 97, 4, 5]
>>> L[3]
94
>>> L[7]
4
>>> L[3:7] = []
>>> L
[0, 1, 93, 4, 5]

Similarly, you can insert elements into a sequence by using an empty slice on the left-hand side.

>>> L
[0, 1, 93, 4, 5]
>>> L[2]
93
>>> L[2:2] = [41, 43, 47, 53]
>>> L
[0, 1, 41, 43, 47, 53, 93, 4, 5]

Another way to delete elements from a sequence is to use Python's del statement.

>>> L
[0, 1, 41, 43, 47, 53, 93, 4, 5]
>>> L[3:6]
[43, 47, 53]
>>> del L[3:6]
>>> L
[0, 1, 41, 93, 4, 5]