[Python-3000] Iterators for dict keys, values, and items == annoying :)

[Ian Bicking]

Guido van Rossum wrote:
> On 3/23/06, Ian Bicking <ianb at colorstudy.com> wrote:
> 
>>This has been my personal experience with the iterators in SQLObject as
>>well.  The fact that an empty iterator is true tends to cause particular
>>problems in that case, though I notice iterkeys() acts properly in this
>>case; maybe part of the issue is that I'm actually using iterables
>>instead of iterators, where I can't actually test the truthfulness.
> 
> 
> This sounds like some kind of fundamental confusion -- you should
> never be tempted to test an iterator for its truth value.

I'm testing if it is empty or not, which seems natural enough.  Or would 
be, if it worked.  So I start out doing:

   for item in select_results: ...

Then I realize that the zero-item case is special (which is common), and do:

   select_results = list(select_results)
   if select_results:
       ...
   else:
       for item in select_results:...

That's not a very comfortable code transformation.  When I was just 
first learning Python I thought this would work:

   for item in select_results:
       ...
   else:
       ... stuff when there are no items ...

But it doesn't work like that.


.iterkeys() does return an iterator with a useful __len__ method, so the 
principle that iterators shouldn't be tested for truth doesn't seem right.

(Very small mostly unrelated problem that occurs to me just at this 
moment -- I can't override __len__ with any implementation that isn't 
really cheap, because lots of code calls __len__ under the covers, like 
list() -- originally SQLObject used len(query) to do a COUNT(*) query, 
but that didn't work)

>>Another issue I have with generators (and hence iterators) is the
>>uselessness of repr() on them.  This causes a lot of list() invocations
>>as well, which works with interactive testing, but fails badly with
>>print statements (where you have to do another run with list() around it
>>-- but that itself causes problems when exhausting the iterator
>>introduces a new bug).
> 
> 
> Views could solve this problem as well, since they are reiterable.

Yes, I would expect them to have a good __repr__, and so it wouldn't be 
a problem.

-- 
Ian Bicking  /  ianb at colorstudy.com  /  http://blog.ianbicking.org