[Python-Dev] PEP 218 update questions

[A.M. Kuchling]

I've just updated PEP 218 to match the sets.py module that's in CVS.  

The original PEP text had a single Set class, and Set instances froze
after their __hash__ was called so you could use a set as a dictionary
key.  sets.py takes a more straightforward approach, with distinct Set
and ImmutableSet classes.

Two things in the PEP need to be updated to take this change into account.

1)
The long-term proposal in PEP 218 lists a single built-in conversion
function, set(iterable).  This made sense when there was a single Set
class, but now this needs to be updated.  

How would instances of a built-in immutable set type be created?
Would there be a second immutable_set() built-in, or would the set()
function take an additional argument: set(iterable, immutable=True)?

2)
The PEP proposes {1,2,3} as the set notation and {-} for the empty
set.  Would there be different syntax for an immutable and a mutable
set?  

My thought is, "no"; {1,2,3} or {-} is always mutable, and an
immutable set would be written as immutable_set({1,2,3}).  That's up
to python-dev, though, but whatever gets chosen needs to be specified
in the PEP.

--amk