set and dict iteration

[Steven D'Aprano]

On Thu, 23 Aug 2012 09:49:41 -0700, Aaron Brady wrote:

[...]
> The patch for the above is only 40-60 lines.  However it introduces two
> new concepts.
> 
> The first is a "linked list", a classic dynamic data structure, first
> developed in 1955, cf. http://en.wikipedia.org/wiki/Linked_list . 
> Linked lists are absent in Python

They certainly are not. There's merely no named "linked list" class.

Linked lists are used by collections.ChainMap, tracebacks, xml.dom, 
Abstract Syntax Trees, and probably many other places. (Well, technically 
some of these are trees rather than lists.) You can trivially create a 
linked list:

x = [a, [b, [c, [d, [e, None]]]]]

is equivalent to a singly-linked list with five nodes. Only less 
efficient.


> The second is "uncounted references".  The uncounted references are
> references to "set iterators" exclusively, exist only internally to
> "set" objects, and are invisible to the rest of the program.  The reason
> for the exception is that iterators are unique in the Python Data Model;
> iterators consist of a single immutable reference, unlike both immutable
> types such as strings and numbers, as well as container types.  Counted
> references could be used instead, but would be consistently wasted work
> for the garbage collector, though the benefit to programmers' peace of
> mind could be significant.

The usual way to implement "uncounted references" is by using weakrefs. 
Why invent yet another form of weakref?



-- 
Steven