Merge sort with inversion counting

inversions.py

ints = [int(x) for x in open('IntegerArray.txt').read().split()]

def merge_and_count(a, b):
    assert a == sorted(a) and b == sorted(b)
    c = []
    count = 0
    i, j = 0, 0
    while i < len(a) and j < len(b):
        c.append(min(b[j], a[i]))
        if b[j] < a[i]:
            count += len(a) - i 
            j+=1
        else:
            i+=1
    c += a[i:] + b[j:]
    return count, c

def sort_and_count(li):
    if len(li) == 1: return 0, li
    n = len(li) // 2 
    a, b = li[:n], li[n:]
    ra, a = sort_and_count(a)
    rb, b = sort_and_count(b)
    r, li = merge_and_count(a, b)
    return ra+rb+r, li

print sort_and_count(ints)[0]