A (kind of) functional implementation of the merge sort algorithm in Python.

mergesort.py

def merge(a, b):
    if not a:
        return b
    if not b:
        return a
    return a[:1] + merge(a[1:], b) if a[0] < b[0] else b[:1] + merge(a, b[1:])

def mergesort(a):
    len_a = len(a)
    if len_a == 1:
        return a
    k = len_a / 2
    return merge(mergesort(a[:k]), mergesort(a[k:]))

if __name__ == '__main__':
    import random
    random.seed()
    a = range(30)
    random.shuffle(a)
    print mergesort(a)