Question

Let’s say I have a table of values in Python. I generate the table this way:

table = [[True] * 11] * 7

This makes for a list of 7 lists, each of which contain 11 True values:

[[True True True True True True True True True True True],
 [True True True True True True True True True True True],
 [True True True True True True True True True True True],
 [True True True True True True True True True True True],
 [True True True True True True True True True True True],
 [True True True True True True True True True True True],
 [True True True True True True True True True True True]]

Say I change table[6][4] to False (table[6][4] = False). I would expect only that slot to be changed:

[[True True True True True  True True True True True True],
 [True True True True True  True True True True True True],
 [True True True True True  True True True True True True],
 [True True True True True  True True True True True True],
 [True True True True True  True True True True True True],
 [True True True True True  True True True True True True],
 [True True True True False True True True True True True]]

But apparently the 4th column in every other list is changed as well:

[[True True True True False True True True True True True],
 [True True True True False True True True True True True],
 [True True True True False True True True True True True],
 [True True True True False True True True True True True],
 [True True True True False True True True True True True],
 [True True True True False True True True True True True],
 [True True True True False True True True True True True]]

What’s going on?

Answer 1

If you do

table = [[True] * 11] * 7

you create a list with 11 Trues and then you wrap it seven-fold in a list. Not by copying it, but by referencing it several times.

If you create 7 distinct lists, such as in

table = [[True] * 11 for _ in range(7)]

you are better off.