>>> def matrix(x,y):
...     a = list()
...     for rows in xrange(y):
...             a.append(list())
...             for cols in xrange(x):
...                     a[rows].append(0)
...     return a
... 
>>> a = matrix(3,3)
>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> a[1][1] = 3
>>> a
[[0, 0, 0], [0, 3, 0], [0, 0, 0]]