Question

Given a list of lists:

>>> n=4
>>> LoL=[range(n) for i in range(n)]
>>> LoL
[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

Is it readily apparent, understandable, Pythonic even to assure a N x N matrix this way:

>>> len(LoL) == n and {len(l) for l in LoL} == {n}
True

So it would be used thus:

if len(matrix) != 4 or {len(l) for l in matrix} != {4}:
        raise ValueError

Is there a better alternative idiom or is this understandable?

Answer 1

As stated in your comment, try / except is probably better.

Not only will you catch and attempt to use an element outside of the 4×4 size, but you will also catch the wrong dimensions passed to you:

>>> LoL=[1,2,3,4]
>>> len(LoL) == n and {len(l) for l in LoL} == {n}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <setcomp>
TypeError: object of type 'int' has no len()

Vs, if you have doubt about the data you are about to use:

>>> try: 
...    i=LoL[2][2]
... except IndexError:
...    print 'no bueno...'
... 
no bueno...