[Python-ideas] set.add could return True or False

set.add(x) could return True if x was added to the set, and False if x was already in the set.

Adding an element that is already present often constitutes an error in my code.

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.