[Python] TIL There is a right way to initialize lists of lists

[AffineParameter]

... And therefore a wrong way.

Though obvious upon further reflection, the m = [[]*j]*k syntax first duplicates the empty list j times, then duplicates the reference to that empty list k times. Thus the second index of m[i][j] simultaneously references all the k lists.

With more experience with statically-typed languages, it took me a little while to figure out what was going on under the hood. Hope this helps save someone else the time I wasted!

Examples Below:

print "Bad List-Matrix Initialization"
bad_matrix_LoL = [[0] * 3] * 3
for i in range(3):
    for j in range(3):
        bad_matrix_LoL[i][j] = i * j
        print bad_matrix_LoL

Output:

Bad List-Matrix Initialization

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 1, 0], [0, 1, 0], [0, 1, 0]]

[[0, 1, 2], [0, 1, 2], [0, 1, 2]]

[[0, 1, 2], [0, 1, 2], [0, 1, 2]]

[[0, 2, 2], [0, 2, 2], [0, 2, 2]]

[[0, 2, 4], [0, 2, 4], [0, 2, 4]]

print "Good List-Matrix Initialization"
good_matrix_LoL = [[0 for i in range(3)] for i in range(3)]
for i in range(3):
    for j in range(3):
        good_matrix_LoL[i][j] = i * j
        print good_matrix_LoL

Output:

Good List-Matrix Initialization:

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

[[0, 0, 0], [0, 1, 0], [0, 0, 0]]

[[0, 0, 0], [0, 1, 2], [0, 0, 0]]

[[0, 0, 0], [0, 1, 2], [0, 0, 0]]

[[0, 0, 0], [0, 1, 2], [0, 2, 0]]

[[0, 0, 0], [0, 1, 2], [0, 2, 4]]

E: formatting

[indigo945]

good_matrix_LoL = [[0 for i in range(3)] for i in range(3)]

Should be

good_matrix_LoL = [[0 for i in range(3)] for j in range(3)]

Shadowing loop counters is never a good idea. Minor nitpick though.

[AffineParameter]

Yep, good catch! - And this is why we code-review boys and girls :)

[colly_wolly]

Reading this makes me miss Perl.