set and dict iteration

[Ian Kelly]

On Thu, Aug 16, 2012 at 4:55 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi at gmail.com> wrote:
>> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:
>
> It can be more than just the new element.  For example, here the
> entire set is repeated (Python 3.2):
>
>>>> s = set(range(8, 13))
>>>> it = iter(s)
>>>> from itertools import islice
>>>> list(islice(it, 5))  # avoid exhausting the iterator
> [8, 9, 10, 11, 12]
>>>> s.add(13)
>>>> s.remove(13)
>>>> list(it)
> [8, 9, 10, 11, 12]
>
> This occurs because the addition of the sixth item triggers a resize
> of the underlying hash table, and the existing items, which were
> originally in slots 0-4, are now in slots 8-12.

Another curious example:

>>> s = set(range(8, 48, 8))
>>> s
{8, 16, 40, 24, 32}
>>> it = iter(s)
>>> from itertools import islice
>>> list(islice(it, 4))
[8, 16, 40, 24]
>>> s.add(48)
>>> s.remove(48)
>>> list(it)
[8, 16, 40, 24]

Hey, what happened to 32?