Use of "or" in conjunction with update for setting variables

sexy_or.py

d = dict()
d['a'] = 1

d.update(
    {
        'a': 2
    }
)
print d

f = None
d.update(
    {
        'a': f or 3
    }
)
print d

f = 5
d.update(
    {
        'a': f or 4
    }
)
print d
# {'a': 2}
# {'a': 3}
# {'a': 5}