Reverse Indexing in Python?

Reverse Indexing in Python?

You can assign your variable to None:

>>> a = range(20)
>>> a[15:None:-1]
[15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> 

Omit the end index:

print a[15::-1]

Reverse Indexing in Python?

In Python2.x, the simplest solution in terms of number of characters should probably be :

>>> a=range(20)

>>> a[::-1]
[19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Though i want to point out that if using xrange(), indexing wont work because xrange() gives you an xrange object instead of a list.

>>> a=xrange(20)
>>> a[::-1]
Traceback (most recent call last):
  File <stdin>, line 1, in <module>
TypeError: sequence index must be integer, not slice

After in Python3.x, range() does what xrange() does in Python2.x but also has an improvement accepting indexing change upon the object.

>>> a = range(20)
>>> a[::-1]
range(19, -1, -1)
>>> b=a[::-1]
>>> for i in b:
...     print (i)
... 
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
>>> 

the difference between range() and xrange() learned from source: http://pythoncentral.io/how-to-use-pythons-xrange-and-range/
by author: Joey Payne