Mergesort with Python

I couldn’t find any working Python 3.3 mergesort algorithm codes, so I made one myself. Is there any way to speed it up? It sorts 20,000 numbers in about 0.3-0.5 seconds

def msort(x):
    result = []
    if len(x) < 2:
        return x
    mid = int(len(x)/2)
    y = msort(x[:mid])
    z = msort(x[mid:])
    while (len(y) > 0) or (len(z) > 0):
        if len(y) > 0 and len(z) > 0:
            if y[0] > z[0]:
                result.append(z[0])
                z.pop(0)
            else:
                result.append(y[0])
                y.pop(0)
        elif len(z) > 0:
            for i in z:
                result.append(i)
                z.pop(0)
        else:
            for i in y:
                result.append(i)
                y.pop(0)
    return result

Answer

You can initialise the whole result list in the top level call to mergesort:

result = [0]*len(x)   # replace 0 with a suitable default element if necessary. 
                      # or just copy x (result = x[:])

Then for the recursive calls you can use a helper function to which you pass not sublists, but indices into x. And the bottom level calls read their values from x and write into result directly.

That way you can avoid all that poping and appending which should improve performance.