Question

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

Answer 1

You can’t serialize that as json, json has a much less flexible idea about what counts as a dict key than python.

You could transform the mapping into a sequence of key, value pairs, something like this:

import json
def remap_keys(mapping):
    return [{'key':k, 'value': v} for k, v in mapping.iteritems()]
... 
json.dumps(remap_keys({(1, 2): 'foo'}))
>>> '[{"value": "foo", "key": [1, 2]}]'