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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