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Re: Dictionary Views -- good examples? [was Re: Python 3 dict question]

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On Fri, May 6, 2011 at 4:49 PM, Ethan Furman <ethan@stoneleaf.us> wrote:
> Ian Kelly wrote:
>>
>> On Fri, May 6, 2011 at 1:57 PM, dmitrey <dmitrey15@gmail.com> wrote:
>>>
>>> Unfortunately, it doesn't work, it turn out to be dict_items:
>>>>>>
>>>>>> next({1:2}.items())
>>>
>>> Traceback (most recent call last):
>>>  File "<stdin>", line 1, in <module>
>>> TypeError: dict_items object is not an iterator
>>
>> So call iter() on it first:
>>
>> next(iter(myDict.items()))
>
> Which is becoming less elegant.

That's not even the worst of it.  If the dict is empty, then the code
above will raise a StopIteration, which must be caught locally since
if it propagates it could be swallowed by a generator.  So a full
recipe should really look more like this:

try:
    first_item = next(iter(my_dict.items()))
except StopIteration:
    raise ValueError("empty dict")

> Seems to me that View objects should be directly iterable

They are.  They're just not directly nextable because they're treated
as dependent collections, not iterators.

> but then I don't really understand the motivation behind them or what greatness is facilitated by having them.
>
> Anybody care to chime in with their usage of this construct?

You should start with PEP 3106.  The main idea is that dict.keys() and
dict.items() can be treated as frozensets, while still being more
lightweight than lists.  That lets you do nifty things like "a.keys()
== b.keys()" which, if a and b are Python 3 dicts, will tell you
whether they contain the same keys.

Whether anybody actually uses this, I have no idea.

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Python 3 dict question dmitrey <dmitrey15@gmail.com> - 2011-05-06 12:40 -0700
  Re: Python 3 dict question Chris Rebert <clp2@rebertia.com> - 2011-05-06 12:51 -0700
    Re: Python 3 dict question dmitrey <dmitrey15@gmail.com> - 2011-05-06 12:57 -0700
      Re: Python 3 dict question Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-06 14:48 -0600
      Dictionary Views -- good examples?  [was Re: Python 3 dict question] Ethan Furman <ethan@stoneleaf.us> - 2011-05-06 15:49 -0700
        Re: Dictionary Views -- good examples?  [was Re: Python 3 dict question] Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-05-07 21:09 +1200
          Re: Dictionary Views -- good examples?  [was Re: Python 3 dict question] Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-05-08 11:23 +0200
      Re: Dictionary Views -- good examples? [was Re: Python 3 dict question] Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-06 18:23 -0600
      Re: Dictionary Views -- good examples? [was Re: Python 3 dict question] Ethan Furman <ethan@stoneleaf.us> - 2011-05-10 10:18 -0700
  Re: Python 3 dict question Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-06 14:02 -0600
  Re: Python 3 dict question Rob Wolfe <rw@smsnet.pl> - 2011-05-06 22:46 +0200
  Re: Python 3 dict question nirinA raseliarison <nirina.raseliarison@gmail.com> - 2011-05-06 14:18 -0700
  Re: Python 3 dict question Raymond Hettinger <python@rcn.com> - 2011-05-10 10:25 -0700

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