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 chang

Answer 1

This one is faster than the accepted answer!
Using xrange(rows) instead of [0]*rows makes no difference.

>>> from itertools import repeat
>>> rows,cols = 3,6
>>> a=[x[:] for x in repeat([0]*cols,rows)]

A variation that doesn't use itertools and runs around the same speed

>>> a=[x[:] for x in [[0]*cols]*rows]

From ipython:

In [1]: from itertools import repeat

In [2]: rows=cols=10

In [3]: timeit a = [[0]*cols for _ in [0]*rows]
10000 loops, best of 3: 17.8 us per loop

In [4]: timeit a=[x[:] for x in repeat([0]*cols,rows)]
100000 loops, best of 3: 12.7 us per loop

In [5]: rows=cols=100

In [6]: timeit a = [[0]*cols for _ in [0]*rows]
1000 loops, best of 3: 368 us per loop

In [7]: timeit a=[x[:] for x in repeat([0]*cols,rows)]
1000 loops, best of 3: 311 us per loop