Merge sort implementation in Python

mergesort.py

def merge(lblock, rblock):
    li, ri, llen, rlen = 0, 0, len(lblock), len(rblock)
    merged = []
    for _ in range(llen + rlen):
        if li < llen and (ri >= rlen or lblock[li] < rblock[ri]):
            merged.append(lblock[li])
            li += 1
        else:
            merged.append(rblock[ri])
            ri += 1
    return merged

mergesort = lambda data: (merge(mergesort(data[0:len(data)//2]), mergesort(data[len(data)//2:])) if len(data) >= 2 else data)