[Tutor] <mutable>.__mul__

[eryksun]

On Sat, Aug 3, 2013 at 10:50 AM, Albert-Jan Roskam <fomcl at yahoo.com> wrote:
> # repeating items of a list using itertools.repeat
>
>>>> from itertools import repeat
>>>> yyy = repeat([6], 4)
>>>> yyy
> repeat([6], 4)
>>>> yyy = list(yyy)
>>>> yyy
> [[6], [6], [6], [6]]
>>>> yyy[0][0] = 666
>>>> yyy
> [[666], [666], [666], [666]]
> # same thing: assignment of one item changes all items.

You repeated the same list object 4 times in a new list. All that does
is increment the reference count on the original list:

    >>> base = [6]
    >>> sys.getrefcount(base)
    2
    >>> seq = list(repeat(base, 4))
    >>> sys.getrefcount(base)  # +4
    6

You'd need to make a shallow copy:

    >>> base = [6]
    >>> seq = map(list, repeat(base, 4))
    >>> sys.getrefcount(base)
    2
    >>> seq[0][0] = 666
    >>> seq
    [[666], [6], [6], [6]]