[Tutor] Iterating a dict with an iteration counter? How would *you* do it?

[Steven D'Aprano]

On 05/02/13 22:27, Oscar Benjamin wrote:
> On 5 February 2013 03:56, eryksun<eryksun at gmail.com>  wrote:
>> On Mon, Feb 4, 2013 at 7:04 PM, Dave Angel<davea at davea.name>  wrote:
>>>> Nope, in both Python 2 and 3 iterating over a dict directly just
>>>> provides the key. That's also how "if key in dict" works.
>>
>> A dict implements __contains__ for an efficient "in" test. In general,
>> the interpreter falls back to using iteration if a type lacks
>> __contains__.
>
> I almost wrote this response but then I realised that Dave probably
> meant that "obj in dict" returns True if the dict has a key equal to
> obj rather than if the dict has a (key, value) pair equal to obj.


It was actually me, not Dave, and yes, that's what I meant. I didn't mean
that dict containment tests were literally implemented by iterating over
the keys checking each one in turn, since that would be horribly
inefficient for something so critical as a dict. Although I can see why
eryksun may have thought so, sorry for any confusion caused by my poor
wording.

Although note that Python does fallback on iteration for containment if
you don't define a __contains__ method:


py> class Test(object):
...     def __getitem__(self, n):
...             if n >= 5: raise IndexError
...             return n + 100
...
py> t = Test()
py> 3 in t
False
py> 103 in t
True



-- 
Steven