Serializing a nested namedtuple into JSON with Python >= 2.7

Question

I have a problem similar to CalvinKrishy's problem Samplebias's solution is not working with the data I have.

I am using Python 2.7.

Here's the data:

Namedtuple

>>> a_t = namedtuple('a','f1 words')
>>> word_t = namedtuple('word','f2 value')
>>> w1 = word_t(f2=[0,1,2], value='abc')
>>> w2 = word_t(f2=[3,4], value='def')
>>> a1 = a_t(f1=[0,1,2,3,4],words=[w1, w2])
>>> a1
a(f1=[0, 1, 2, 3, 4], words=[word(f2=[0, 1, 2], value='abc'), word(f2=[3, 4], value='def')])

Dict

>>> w3 = {}
>>> w3['f2'] = [0,1,2]
>>> w3['value'] = 'abc'
>>> w4 = {}
>>> w4['f2'] = [3,4]
>>> w4['value'] = 'def'
>>> a2 = {}
>>> a2['f1'] = [0, 1, 2, 3, 4]
>>> a2['words'] = [w3,w4]
>>> a2
{'f1': [0, 1, 2, 3, 4], 'words': [{'f2': [0, 1, 2], 'value': 'abc'}, {'f2': [3, 4], 'value': 'def'}]}

As you can see that both a1 and a2 are same except that one is namedtuple and other is dict.

But the json.dumps is different:

>>> json.dumps(a1._asdict())
'{"f1": [0, 1, 2, 3, 4], "words": [[[0, 1, 2], "abc"], [[3, 4], "def"]]}'
>>> json.dumps(a2)
'{"f1": [0, 1, 2, 3, 4], "words": [{"f2": [0, 1, 2], "value": "abc"}, {"f2": [3, 4], "value": "def"}]}'

I want to have json format of a1 exactly like its doing for a2.