how can we riffle shuffle the elements of a list in python?

Question

I want to shuffle the elements of a list without importing any module. The type of shuffle is riffle shuffle. Where you want to divide the no. of elements of the list into two and then interleave them.

if there are odd no. of elements then the second half should have contain the extra element

eg: list = [1,2,3,4,5,6,7]

then the final list should look like [1,4,2,5,3,6,7]

Answer 1

Just for fun, a recursive solution:

def interleave(lst1, lst2):
    if not lst1:
        return lst2
    elif not lst2:
        return lst1
    return lst1[0:1] + interleave(lst2, lst1[1:])

Use it as follows in Python 2.x (In Python 3.x, use // instead of /):

lst = [1,2,3,4,5,6,7]
interleave(lst[:len(lst)/2], lst[len(lst)/2:])
=> [1, 4, 2, 5, 3, 6, 7]

The above will work fine with lists of any length, it doesn't matter if the length is even or odd.