Setting default_factory of defaultdict to key

[Larry Martell]

Is there a way to set the default_factory of defaultdict so that
accesses to undefined keys get to set to the key?

i.e. if d['xxx'] were accessed and there was no key 'xxx' then
d['xxx'] would get set to 'xxx'

I know I can define a function with lambda for the default_factory,
but I don't see how to access the key. I tried:

d = defaultdict(lambda: key)

But that did not work.