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

[Adam DePrince]

On Fri, 2006-03-24 at 18:58 -0800, Guido van Rossum wrote:
> On 3/24/06, Tim Peters <tim.peters at gmail.com> wrote:
> > [Guido]
> > > The Java collections framework actually has an API and an idiom that
> > > make this work: therd's a method on the *iterator* that deletes the
> > > current item without disturbing the iteration.
> >
> > Yup, I saw that:
> >
> >     http://java.sun.com/j2se/1.4.2/docs/api/java/util/Iterator.html
> >
> > It's at least curious that there isn't also a method to add an item in
> > the base Iterator interface.
> 
> I could ask my drinking buddy Josh Bloch, but I'll try to channel him
> first: (a) the use case isn't as attractive (why would you want to
> insert an item during an iteration? the use case for deletion during
> iteration is very natural); (b) did you mean to insert it before,
> after, or a random item? for hash sets that doesn't matter, but for
> trees and lists it does. (c) it would be hard to implement for hash
> sets because you may not have a choice about not rehashing.
> 
> [...]
> > > Note that this is all quite independent from the views proposal (also
> > > inspired by Java).
> >
> > I'm not sure I saw a coherent "views proposal" ;-)  Java has some nice
> > ideas, but I'd think it's too elaborate for your tastes (and, by
> > extension, for Python's).  For example, it has six(?) collection
> > interfaces, two of which don't even extend the base Collection
> > interface.  AFAICT, what Java calls "views" are unique to some methods
> > of its Map and SortedMap interfaces.
> 
> Right, the views only make sense for maps that have separate keys and values.
> 
> > To be concrete, is what you're calling "a view" the kind of thing
> > returned by Java's AbstsactMap.entrySet()?:
> >
> >     http://java.sun.com/j2se/1.4.2/docs/api/java/util/AbstractMap.html
> 
> Yes. Another example is Map.keySet(), which returns a set view of the
> keys of a map:
> 
> http://java.sun.com/j2se/1.4.2/docs/api/java/util/Map.html#keySet()
> 
> > That's closest to Python's items(), and AbstractMap.keys() is closest
> > to Python's keys().  Java being Java, AbstractMap.values() returns a
> > different _type_ of object than those two (Java's further
> > sub-distinction between "set views" and "collection views").
> 
> The "Abstract" classes aren't the most interesting pieces; they are
> really building blocks for implementers, as explained here:
> 
> http://java.sun.com/j2se/1.4.2/docs/guide/collections/overview.html
> (section "Collection Implementations", last para of the intro).
> 
> The really core stuff is in the interfaces.
> 
> The difference between sets and collections is that collections may
> have duplicates; that's pretty essential for the thing you get from
> values(), but keys() and items() return sets (of different types).
> 
> > I vote to steal the good parts and drop all the distinctions :-)
> 
> Maybe. I need a volunteer to write the PEP!

Oh, why not.  Me me! 

- Adam