[Python-3000] Set literal

[Guido van Rossum]

Looking over the code base, frozensets are used rarely. So I don't
think this is warranted.

On Jan 24, 2008 2:32 PM, Raymond Hettinger <python at rcn.com> wrote:
> I think it would be more useful for the {e1, e2, e3} literal to be a frozenset instead of a set.
>
> In expressions like "x in {'html', 'xml', 'php'}" the compiler could optimize away the set construction and treat it as a constant.
>
> In cases where we want to build-up mutable sets, we need to start with set() anyway:
>
>     s = set()
>     for elem in source:
>         s.add(elem)
>
> I don't think it would be typical to start with a partially filled-out set and then build-t up further:
>
>    s = {'a', 'b', 'c'}   # rare use-case
>    for elem in source:
>        s.add(elem)
>
> One of the primary use cases for frozensets is to be members of other sets or to be keys in a dict (esp. for graph representations).  The repr's of those nested structures are annoying to read because the word "frozenset" gets spelled-out over and over again.  Here's a few lines from the pprint() output for a graph of a cube:
>
>   {frozenset([0, 1]): frozenset([frozenset([0]),
>                                  frozenset([1]),
>                                  frozenset([0, 1, 2])]),
>    frozenset([0, 1, 2]): frozenset([frozenset([1, 2]),
>                                     frozenset([0, 2]),
>                                     frozenset([0, 1])])}
>
>
> This would read *much* better with the new notation:
>
>   {{0, 1}: {{0},
>             {1},
>             {0, 1, 2}},
>   {0, 1, 2}: {{1, 2},
>               {0, 2},
>               {0, 1}}}
>
> If you want to see how extremely bad the current repr's can get, see the repr for David Eppstein's cuboctahedron in the tests for pprint:  http://mail.python.org/pipermail/python-checkins/2008-January/065099.html
>
> In short, I think we would be much better served by using the {} literal notation for frozensets.
>
> Raymond
>
>
> P.S.  A small side-benefit is it may put an end for interminable requests for a {:} or {/} notation for empty sets.  There's not much need for a literal for a empty frozenset (use "not s" instead).
>
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--Guido van Rossum (home page: http://www.python.org/~guido/)