Just realized that I failed to send this to the list as well: On Tue, Jun 3, 2008 at 3:48 PM, Raymond Hettinger <python@rcn.com> wrote:
That's silly. Lot's of functions do odd things with random argument ordering:
s = set([9, 3]) int.__sub__(*s)
6
Besides, you can already run the sample fragment in Py2.5:
sos = set( [frozenset([1, 2]), frozenset([2, 3])]) frozenset.difference(*sos)
frozenset([1])
Raymond
Right, but that's why these functions do not accept more than two arguments. They are intended to be used as instance methods only. If we're promoting the idea of a set.method(*args) usage, however, the usage should probably be intuitive. Because they are associative, union, intersection, and symmetric_difference are all intuitive and do what is expected no matter what. That is not true of set-difference. In other words, it looks to me like set.method(*args) is trying to say "Take all of elements in these iterables and make one set out of them," or more simply, "Throw all this crap together." Intuitively, it _shouldn't_ matter what order the arguments are in. What is the meaning of taking the set-difference of a bunch of sets? Should we promote an operation that doesn't make any sense? In mathematics, there are symbols for set.union(*args) (big-U) and set.intersection(*args) (big-upside-down-U), because they actually come up in common usage. I'm not aware of any such symbols for other set operations. Now that doesn't necessarily mean we shouldn't support them, but it is certainly something to think about. To take it from another angle, it is easy to define: set.union(*args) - the set of all the elements appearing in at least one of the args set.intersection(*args) - the set of all the elements appearing in every arg set.symmetric_difference(*args) - the set of all the elements appearing in an odd number of arguments but: set.difference(*args) - the set of all elements appearing in the first arg but not any of the rest is fundamentally different. When using set operations, ordering shouldn't even be a consideration. However, A.difference(*args) - the set of all elements in A that do not appear in any of the args is well-defined. For that reason, I say that we should support *args for all set operations, but we should only promote the use of set.method syntax for intersection and union. set.difference doesn't seem well-defined, and set.symmetric_difference doesn't seem very useful (and could lead to usage of set.difference). So... +1 supporting *args for all set operations +1 documenting the usage of set.union(*args) and set.intersection(*args) as unioning/intersecting all of the arguments -1 even mentioning set.difference or set.symmetric_difference in static usage That's my 2c, Brandon