Anyone remember why setdefault's second argument is optional?
d = {} d.setdefault(666) d {666: None}
just doesn't seem useful. In fact, it's so silly that someone calling setdefault with just one arg seems far more likely to have a bug in their code than to get an outcome they actually wanted. Haven't found any 1-arg uses of setdefault() either, except for test code verifying that you _can_ omit the second arg. This came up in ZODB-land, where someone volunteered to add setdefault() to BTrees. Some flavors of BTrees are specialized to hold integer or float values, and then setting None as a value is impossible. I resolved it there by making BTree.setdefault() require both arguments. It was a surprise to me that dict.setdefault() didn't also require both. If there isn't a sane use case for leaving the second argument out, I'd like to drop the possibility in P3K (assuming setdefault() survives).