On Mar 15, 2012 1:53 AM, "Masklinn" <email@example.com> 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?
Because dict insertions are by operator?
> > Adding an element that is already present often constitutes an error in my code.
> Then thrown an error when that happens?
> > As I understand, set.add is an atomic operation. Having set.add return
> > a boolean will also allow EAFP-style code with regard to handling
> > duplicates, the long winded form of which is currently:
> > if a not in b:
> > b.add(a) <-- race condition
> > do_c()
> > Which can be improved to:
> > if b.add(a):
> > do_c()
> > Advantages:
> > * Very common code pattern.
> > * More concise.
> > * Allows interpreter atomicity to be exploited, often removing the
> > need for additional locking.
> > * Faster because it avoids double contain check, and can avoid locking.
> Nope, as Andrew noted it's possible that an other thread has *removed*
> the element from the set before do_c (so you've got your race condition
> right there, assuming do_c expects `a` to be in `b`)
> And since you're using a set `b.add(a)` is a noop if `a` is already in
> `b` so there's no race condition at the point you note. The race
> condition is instead in the condition itself, you can have two different
> threads finding out the value is not in the set and ultimately executing
There's still a performance cost in looking up the already present value a second time.