python list comprehension: creating 2d array

Question

Are these two expressions the same?

a = [[0]*3]*3
b = [[0]*3 for i in range(3)]

The resulting a and b values look the same. But would one way be better than the other? What is the difference here.

Answer 1

They're not the same

>>> a[1][2] = 5
>>> a
>>> [[0, 0, 5], [0, 0, 5], [0, 0, 5]]


>>> b[1][2] = 5
>>> b
>>> [[0, 0, 0], [0, 0, 5], [0, 0, 0]]

The first one creates an outer array of pointers to a single inner array while the second actually creates 3 separate arrays.