What is wrong with the following program code, attempting to initialize a 4 x 4 matrix of integers? How should the initialization be done?

line = [0] * 4
matrix = [line, line, line, line]

Use a list comprehension:

>>> line = [[0]*4 for _ in xrange(4)]
>>> line
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

Don't do this though:

>>> line = [[0]*4]*4
>>> line
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

The output looks same, but the problem here is all inner lists are actually the same object repeated 4 times:

>>> [id(x) for x in line]
[156931756, 156931756, 156931756, 156931756]

So, changing one of them is going to affect all:

>>> line[2][0] = 10
>>> line
[[10, 0, 0, 0], [10, 0, 0, 0], [10, 0, 0, 0], [10, 0, 0, 0]]

Same thing is applicable to your code:

>>> line = [0] * 4
>>> matrix = [line, line, line, line]
>>> [id(x) for x in matrix]
[158521804, 158521804, 158521804, 158521804]

If line contains only immutable object then you can change your code do:

>>> matrix = [line[:] for _ in xrange(4)]

But, if line contains mutable objects itself, then you'd have to use either copy.deepcopy or better create a new line object inside the list comprehension.