I should not have emphasized the atomicity here, this was not intended to be the main reason.

On Mar 15, 2012 2:11 AM, "Chris Rebert" <pyideas@rebertia.com> wrote:
On Wed, Mar 14, 2012 at 10:53 AM, Masklinn <masklinn@masklinn.net> wrote:
> On 2012-03-14, at 18:36 , Matt Joiner wrote:
>> set.add(x) could return True if x was added to the set, and False if x
>> was already in the set.
> That does not mesh with the usual Python semantics of methods either
> having a side-effect (mutation) or returning a value. Why would that
> happen with sets but not with e.g. dicts?

The rule is a bit more complicated than that (e.g. consider
list.pop()). It's gets fleshed out well in:

set.remove() arguably "returns" the same sort of indication as that
which is proposed, in that it either raises or doesn't raise KeyError
depending on whether the value was present.

But yeah, these boolean return values aren't of huge utility,
particularly in the multithreaded case.