why in returns values for array and keys for dictionary

[Erik Max Francis]

Marc 'BlackJack' Rintsch wrote:

> On Mon, 25 Aug 2008 19:57:06 -0700, alex23 wrote:
> 
>> On Aug 26, 10:49 am, "++imanshu" <himanshu.g... at gmail.com> wrote:
>>>     Wouldn't it be nicer to have 'in' return values (or keys) for
>>>     both
>>> arrays and dictionaries. Arrays and Dictionaries looked so similar in
>>> Python until I learned this difference.
>> […]
>>
>> In both cases, 'in' returns a boolean indicating the existence of an
>> item in the list, or a key in the dict. I'm not sure why you'd need it
>> to return the item you're checking for the existence of, as you'd have
>> to have that item before you could do the check.
>>
>> Have I missed what you're asking for here? Could you provide a
>> pseudocode example to demonstrate what you mean?
> 
> The OP isn't talking about the ``in`` operator but ``in`` as part of 
> ``for … in …``.  So it's actually the question why ``list(a_dict)`` 
> doesn't return a list of values but a list of keys.

That is basically the same question.  Iterating over a list gives you 
its elements, and using the `in` operator with lists tells you whether 
or not an object is an element of the list.  Iterating over a dictionary 
gives you its _keys_, not its values, and the `in` operator with 
dictionaries tells you whether or not a _key_ is in the dictionary.

 >>> l = [1, 2, 3]
 >>> for x in l: print x
...
1
2
3
 >>> 0 in l
False
 >>> 1 in l
True
 >>> d = {'a': 1, 'b': 2, 'c': 3}
 >>> for x in d: print x
...
a
c
b
 >>> 'a' in d
True
 >>> 1 in d
False

The reason why people find it more useful to deal with keys rather than 
values of a dictionary during iteration or containment testing is ... 
because that tends to be what you're usually more interested in, and is 
more efficient.  For another thing, if you're doing a lot of testing for 
containment in values, then it's likely you're not using the right data 
structure, or combination of data structures.  That's not what 
dictionaries are for.

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