Python dict.setdefault: Assign only when the key is missing

Want to assign to a Python dict, but only if the key doesn’t yet exist? Use setdefault:

d = {'a':10, 'b':20}

d.setdefault('a', 999) # returns 10, meaning "a already has a value of 10"

d.setdefault('x', 999) # returns 999, meaning "x now has a value of 999"