[Python-Dev] Re: PEP 218 (sets); moving set.py to Lib
Guido van Rossum
Thu, 29 Aug 2002 10:06:53 -0400
> 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):
> if isinstance(iterable, dict):
> . . .
> 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')
> 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 inner.add('z') # now the outer set is out-of-date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])
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/)