Marc 'BlackJack' Rintsch wrote:
On Mon, 25 Aug 2008 19:57:06 -0700, alex23 wrote:
On Aug 26, 10:49 am, "++imanshu" <[EMAIL PROTECTED]> 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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