Inserting a value into all possible locations in a list

Use insert() to insert an element before a given position.

For instance, with

arr = ['A','B','C']
arr.insert(0,'D')

arr becomes ['D','A','B','C'] because D is inserted before the element at index 0.

Now, for

arr = ['A','B','C']
arr.insert(4,'D')

arr becomes ['A','B','C','D'] because D is inserted before the element at index 4 (which is 1 beyond the end of the array).

However, if you are looking to generate all permutations of an array, there are ways to do this already built into Python. The itertools package has a permutation generator.

Here's some example code:

import itertools
arr = ['A','B','C']
perms = itertools.permutations(arr)
for perm in perms:
    print perm

will print out

('A', 'B', 'C')
('A', 'C', 'B')
('B', 'A', 'C')
('B', 'C', 'A')
('C', 'A', 'B')
('C', 'B', 'A')

You could do this with the following list comprehension:

[mylist[i:] + [newelement] + mylist[:i] for i in xrange(len(mylist),-1,-1)]

With your example:

>>> mylist=['A','B']
>>> newelement='X'
>>> [mylist[i:] + [newelement] + mylist[:i] for i in xrange(len(mylist),-1,-1)]
[['X', 'A', 'B'], ['B', 'X', 'A'], ['A', 'B', 'X']]