[Tutor] use of the newer dict types

[eryksun]

On Thu, Jul 25, 2013 at 6:37 PM, Dave Angel <davea at davea.name> wrote:
>
> For dictionaries, instead of returning a list, keys() returns a dictionary
> view.  See http://docs.python.org/3/library/stdtypes.html#dict-views
>
> What's not clear to me is whether this dictionary view object is safe to use
> when the dictionary is changing.  The page referenced above seems to say
> both yes and no.

The view isn't an iterator. A separate iterator gets created for that:

    >>> d = {'a': 1}
    >>> keys = d.keys()
    >>> it = iter(keys)

    >>> d['b'] = 2  # invalidate "it"

    >>> list(it)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    RuntimeError: dictionary changed size during iteration

This creates and iterates a new dict_keyiterator:

    >>> list(keys)
    ['b', 'a']