[Python] JSON serialize a dictionary with tuples as key

Is there a way in Python to serialize a dictionary that is using a tuple as key?

e.g.

a = {(1, 2): 'a'}


simply using json.dumps(a) raises this error:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.6/json/__init__.py", line 230, in dumps
return _default_encoder.encode(obj)
File "/usr/lib/python2.6/json/encoder.py", line 367, in encode
chunks = list(self.iterencode(o))
File "/usr/lib/python2.6/json/encoder.py", line 309, in _iterencode
for chunk in self._iterencode_dict(o, markers):
File "/usr/lib/python2.6/json/encoder.py", line 268, in _iterencode_dict
raise TypeError("key {0!r} is not a string".format(key))
TypeError: key (1, 2) is not a string

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