Why does append() overwrite/clobber every existing element of a list with the one that was just appended?

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This is really a matter of how Python handles assignments of lists. In both of your append commands, Python is pointing to the same list L. This is why the first list that you append appears to change. The append command here does not actually make a new copy of the list L and then put it in M. Instead, both append commands put references to the original list L in the new list M. To append a new copy of the list L, you could use: M.append(copy(L)) or M.append(L[:]).