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Repetition Adds One Level Deep

Repeating a sequence is like adding it to itself a number of times. However, when mutable sequences are nested, the effect might not always be what you expect. For instance, in the following example X is assigned to L repeated four times, whereas Y is assigned to a list containing L repeated four times:

>>> L = [4, 5, 6]
>>> X = L * 4                   # Like [4, 5, 6] + [4, 5, 6] + ...
>>> Y = [L] * 4                 # [L] + [L] + ... = [L, L,...]

>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]
>>> Y
[[4, 5, 6], [4, 5, 6], [4, 5, 6], [4, 5, 6]]

Because L was nested in the second repetition, Y winds up embedding references back to the original list assigned to L, and so is open to the same sorts of side effects noted in the last section:

>>> L[1] = 0                    # Impacts Y but not X
>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]
>>> Y
[[4, 0, 6], [4, 0, 6], [4, 0, 6], [4, 0, 6]]

The same solutions to this problem apply here as in the previous section, as this is really just another way to create the shared mutable object reference case. If you remember that repetition, concatenation, and slicing copy only the top level of their operand objects, these sorts of cases make much more sense.