Merge sorting in Python

Early subject was implemented in C/C++, but actually I do prototyping usually in Python, so this was first :)

def MergerSort(a):
    def MergerGroup(a, left, m, right):
        if left >= right: return None
        if m < left or right < m: return None
        print "left:", left, "m:", m, "right:", right
        t = left
        for j in xrange(m+1, right+1):
            for i in xrange(t, j):
                if a[j] < a[i]:
                    r = a[j]
                    for k in xrange(j, i, -1):
                        a[k] = a[k - 1]
                    a[i] = r
                    t = i
                    break
    if len(a) < 2: return None
    k=1
    while k<len(a):
       g=0
       while g<len(a):
            z = g + k + k - 1
            r = z if z < len(a) else len(a) - 1
            print "k =", k, "g =", g, "z =", z, "r =", r, " | "\
                    "left:", g, "m:", g+k-1, "right:", r
            MergerGroup(a, g, g + k - 1, r)
            g+=2*k
       k*=2

def mergsort(arr):
    arrlen = len(a)
    k = 2
    while k < 2*arrlen:
        left = 0
        while left < arrlen:
            right = min(left + k - 1, arrlen-1)
            print left, right, "|", k
            left += k
        k *= 2

a = [10, 19, 1, 2, 10, 450, 6, 7, 8, 100, 120, 121]
print a, len(a)
MergerSort(a)
print a