Creating lists of lists in a pythonic way

Question

I'm using a list of lists to store a matrix in python. I tried to initialise a 2x3 Zero matrix as follows.

mat=[[0]*2]*3

However, when I change the value of one of the items in the matrix, it changes the value of that entry in every row, since the id of each row in mat is the same. For example, after assigning

mat[0][0]=1

mat is [[1, 0], [1, 0], [1, 0]].

I know I can create the Zero matrix using a loop as follows,

mat=[[0]*2]
for i in range(1,3):
    mat.append([0]*2)

but can anyone show me a more pythonic way?

Answer 1

Use a list comprehension:

>>> mat = [[0]*2 for x in xrange(3)]
>>> mat[0][0] = 1
>>> mat
[[1, 0], [0, 0], [0, 0]]

Or, as a function:

def matrix(rows, cols):
    return [[0]*cols for x in xrange(rows)]