[Python-ideas] Implement __add__ for set and frozenset

[Arnaud Delobelle]

On 3 Jun 2008, at 19:44, Raymond Hettinger wrote:

> > Difference is not an associative operation though. E.g.
>
> A.difference(B, C, D)  means A - B - C - D
> which can be (((A - B) - C) - D)
> or (((A - D) - C) - B)
> or (((A - C) - B) - D)
> or (((A - C) - D) - B)
>
> You can do the subtractions from A in any order.

That's true.  However there no way to predict the output of this:

 >>> set_of_sets = { {1, 2}, {2, 3} }
 >>> set.difference(*set_of_sets)

And

     A.difference(B, C, D)

could be rewritten as

     A - set.union(B, C, D)

which may be just as clear.

-- 
Arnaud