[python] Inserting a value into all possible locations in a list

I am trying to do print all the possible outcomes of a given list and I was wondering how to put a value into various locations in the list. For example, if my list was [A, B], I want to insert X into all possible index of the list such that it would return this [X, A, B], [A, X, B], [A, B, X].

I was thinking about using range(len()) and a for loop but not sure how to start.

Simplest is use list[i:i]

    a = [1,2, 3, 4]
    a[2:2] = [10]

Print a to check insertion

    print a
    [1, 2, 10, 3, 4]

If l is your list and X is your value:

for i in range(len(l) + 1):
    print l[:i] + [X] + l[i:]

Coming from JavaScript, this was something I was used to having "built-in" via Array.prototype.splice(), so I made a Python function that does the same:

def list_splice(target, start, delete_count=None, *items):
    """Remove existing elements and/or add new elements to a list.

    target        the target list (will be changed)
    start         index of starting position
    delete_count  number of items to remove (default: len(target) - start)
    *items        items to insert at start index

    Returns a new list of removed items (or an empty list)
    """
    if delete_count == None:
        delete_count = len(target) - start

    # store removed range in a separate list and replace with *items
    total = start + delete_count
    removed = target[start:total]
    target[start:total] = items

    return removed

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')

If you want to insert a list into a list, you can do this:

>>> a = [1,2,3,4,5]
>>> for x in reversed(['a','b','c']): a.insert(2,x)
>>> a
[1, 2, 'a', 'b', 'c', 3, 4, 5]