[Python-Dev] Re: PEP 218 (sets); moving set.py to Lib

[Guido van Rossum]

> 1. Optimize BaseSet._update(iterable) by checking for two special cases where a C-speed update method is already available and the
> entries are known in advance to be immutable:
> 
>             . . .
>             if isinstance(iterable, BaseSet):
>                 self._data.update(iterable._data)
>                 return
>             if isinstance(iterable, dict):
>                 self._data.update(iterable)
>                 return
>             . . .

Yes.

> 2.  Eliminate the binary sanity checks which verify for operators that 'other' is a BaseSet. If 'other' isn't a BaseSet, try using
> it, directly or by coercing to a set, as an iterable:
> 
> >>> Set('abracadabra') | 'alacazam'
> Set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'])
> 
> This improves usability because the second argument did not have to be pre-wrapped with Set.  It improves speed, for some
> operations, by using the iterable directly and not having to build an equivalent dictionary.

No.  This has been proposed before.  I think it's a bad idea, just as

   [1,2,3] + "abc"

is a bad idea.

If you want this, it's easy enough to do

 s = Set('abracadabra')
 s.update('alacazam')

> 3.  Have ImmutableSet keep a reference to the original iterable.  Add an ImmutableSet.refresh() method that rebuilds ._data from
> the iterable.  Add a Set.refresh() method that triggers ImmutableSet.refresh() where possible.  The goal is to improve the
> usability of sets of sets where the inner sets have been updated after the outer set was created.
> 
> >>> inner = Set('abracadabra')
> >>> outer = Set([inner])
> >>> inner.add('z')                 # now the outer set is out-of-date
> >>> outer.refresh()               # now it is current
> >>> outer
> Set([ImmutableSet(['a', 'c', 'r', 'z', 'b', 'd'])])
> 
> This would only work for restartable iterables -- a file object would not be so easily refreshed.

This *appears* to be messing with the immutability.  If I wrote:

  a = range(3)
  s1 = ImmutableSet(a)
  s2 = Set([s1])
  a.append(4)
  s2.refresh()

What would the value of s1 be?

I think I understand your use case (the example in the docs, where an
employee is added), but I think we should think harder about what to
do about that.  Possibly it's not a good example of how sets are used
(even if it's a good example of how sets work).

--Guido van Rossum (home page: http://www.python.org/~guido/)