[Python-3000] set literals

[Raymond Hettinger]

Guido van Rossum wrote:

>>
>> The deeper problem is that decisions on which values to keep will
>> inevitability break set invariants (see a long list of these in
>> test.test_set.py):
>>
>>    assert a|b == b|a, 'commutative property'
>>    assert (a-b) | (a&b) | (b-a) == a|b, 'whole is the sum of the parts'
>
>
> Yeah, I think when I was cosidering this earlier (it's been a shower
> thought for quite a while on and off) I considered those fatal flaws.
> Today, I think the invariants would simply change to
>
>  assert set(a|b) == set(b|a) == set(a)|set(b) == set(b)|set(a)
>
> with set() being an alias for dict.fromkeys(). 


The outermost set coercion is not especially attractive or efficient.  
Since equality/inequality is an important set operation, we would likely 
need to add a method for equality testing that ignores dict values:

   d1.compareKeys(d2)       # equivalent to:   set(d1)==set(d2)

The frozenset type also needs to be addressed.  They don't come-up much 
in practice but when they do, they are vital to the solution, so some 
kind of frozenset type should be kept around (perhaps in the collections 
module).  However, I have long argued against having frozendicts and I 
want to kill THAT discussion before it rears its ugly head again.

I'm curious as to whether people will find one-type-with-two-purposes 
easier to learn that what we have now.  My experience so far is that 
sets have a near zero learning curve as they are currently implemented.  
Also, I wonder whether it is wise to place any further burden on either 
the mapping API or its implementation (do we want weakref dictionaries 
and dbm's supporting set ops?).


Raymond