How to create and use two Python iterables of shared memory items, and maintain the shared memory during usage?

Question

How can I create a pair of iterables containing the same items (not just copies, but shared memory of items) but in different order, and such that a change to an item in one of the iterables will be reflected in the other iterable?

For example:

>>> x = [0, 1, 2, 3] >>> y = [x[1], x[2], x[3], x[0]] >>> x[2] is y[1] True

Great so far, but if I make a change to one of the items in an iterable, it ends up creating a new item (new memory) space, not changing the item in that same memory space.

>>> x[2] *= 5 >>> x[2] is y[1] False >>> x[2] 10 >>> y[1] 2

Samwise · Accepted Answer · 2021-05-15 16:50:19Z

1

Put each item in a box:

>>> x = [[0], [1], [2], [3]] >>> y = [x[1], x[2], x[3], x[0]] >>> x[2] is y[1] True >>> x[2][0] *= 5 >>> x[2] is y[1] True >>> x[2] [10] >>> y[1] [10]

Here I'm using a list, but any mutable container will do.

answered May 15, 2021 at 16:50

Samwise

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mattroos Over a year ago

Thanks, @Samwise! I had tried something like that before, but with this line instead: x[2] = [x[2][0]*5]. Now I see why that failed. I was creating a new list (what I'll think of unofficially from now on as a "container") rather than replacing the 0th item in the existing list. Great!

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