[Python-Dev] setdefault's second argument

[Tim Peters]

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).