setting default value in a python dictionary

I encountered the following idiom in some OpenERP module recently:

some_dict['key'] = some_dict.get('key', {})

It struck me as an unusual way of setting a default value in a dictionary. So I investigated a bit to compare the performance of the construct compared to other ways of doing the same thing, in order to check if what I would have done (f2 below) was the most efficient way in addition to be the (in my opinion, at least) most readable. I used the timeit module and a few different dictionaries in case the presence of the key influenced the performance of the operation:

import timeit

def f1(d):
    d['value'] = d.get('value', {})

def f2(d):
    if 'value' not in d:
        d['value'] = {}

def f3(d):
    d.setdefault('value', {})

if __name__ == '__main__':
    for d in ('{}', "{'a': None, 'b': None}", "{'a': None, 'b': None, 'value': None}"):
        print d
        for i in (1, 2, 3):
            print "f%d" % i
            t = timeit.Timer('f%d(d)' % i,
                             'from __main__ import f1, f2, f3; d = %s' % d)
            print t.repeat()

With my local version (python 2.7.3), f1 is always the slowest, closely followed by f3, and the readable explicit version (f2) also runs 2x faster.