[Tutor] use of the newer dict types

[Dave Angel]

On 07/25/2013 09:27 PM, eryksun wrote:
> On Thu, Jul 25, 2013 at 7:51 PM, Dave Angel <davea at davea.name> wrote:
>>
>> And it's apparently safe to change the dictionary after creating the view,
>> UNTIL the iterator is created.
>
> Notice the error the iterator raised in my example was just about the
> dict changing size. If the dict changes between steps while remaining
> the same size, the iterator doesn't have a clue:

So in other words, it's unsafe to change the dictionary after creating 
the iterator, but it won't necessarily cause an exception;  it might 
happen to work, or it might just get weird results.

>
>      >>> d = dict.fromkeys('abcdef')
>      >>> d2 = dict.fromkeys('ghijkl')
>
>      >>> keys = d.keys()
>      >>> it = iter(keys)
>      >>> next(it)
>      'f'
>      >>> next(it)
>      'e'
>
>      >>> d.clear()
>      >>> d.update(d2)
>      >>> list(it)
>      ['l', 'k', 'j', 'i', 'h']
>
> 'g' got skipped. The result here depends on the iterator's current
> index into the hash table and the positions that the updated values
> hash to.
>
>> I still don't see the advantage.  Isn't the iterator going to be as big as
>> the list would have been, and take just as long to build?
>
> It already has the hash table. It just has to iterate it looking for
> non-NULL values, and keep track of the previous index between calls to
> __next__().

In other words, no?  The iterator assumes the dict is no longer going to 
change once the iterator is created, so it can just store iteration state.

>
> Another bit of state the iterator maintains is a length hint, which
> helps to optimize memory allocation when using the iterator to create
> a new sequence:
>
>      >>> d = dict.fromkeys('abcdef')
>      >>> it = iter(d)
>      >>> it.__length_hint__()
>      6
>      >>> next(it)
>      'f'
>      >>> it.__length_hint__()
>      5

It'd sure be nice if the library reference actually specified the 
external behavior, though I have no problem with it omitting things like 
__length_hint__.  Those are implementation details.

-- 
DaveA