def pr(a):
    """print matrix a"""
    for r in a:
        for c in r:
            print c,
        print

def addto(x, y):
    """add matrix y to matrix x"""
    rows = len(x)
    cols = len(x[0])
    for i in range(rows):
        for j in range(cols):
            x[i][j] += y[i][j]


a = [[1, 2, 3], [4, 5, 6]]
b = [[10, 10, 10], [20, 20, 20]]
print "a:"; pr(a)
print "b:"; pr(b)

print "a += b"
addto(a, b)
print "a:"; pr(a)