How to dump a Python dictionary to JSON when keys are non-trivial objects?

Question

import datetime, json
x = {'alpha': {datetime.date.today(): 'abcde'}}
print json.dumps(x)

The above code fails with a TypeError since keys of JSON objects need to be strings. The json.dumps function has a parameter called default that is called when the value of a JSON object raises a TypeError, but there seems to be no way to do this for the key. What is the most elegant way to work around this?

Answer 1

You can extend json.JSONEncoder to create your own encoder which will be able to deal with datetime.datetime objects (or objects of any type you desire) in such a way that a string is created which can be reproduced as a new datetime.datetime instance. I believe it should be as simple as having json.JSONEncoder call repr() on your datetime.datetime instances.

The procedure on how to do so is described in the json module docs.

The json module checks the type of each value it needs to encode and by default it only knows how to handle dicts, lists, tuples, strs, unicode objects, int, long, float, boolean and none :-)

Also of importance for you might be the skipkeys argument to the JSONEncoder.

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After reading your comments I have concluded that there is no easy solution to have JSONEncoder encode the keys of dictionaries with a custom function. If you are interested you can look at the source and the methods iterencode() which calls _iterencode() which calls _iterencode_dict() which is where the type error gets raised.

Easiest for you would be to create a new dict with isoformatted keys like this:

import datetime, json

D = {datetime.datetime.now(): 'foo',
     datetime.datetime.now(): 'bar'}

new_D = {}

for k,v in D.iteritems():
  new_D[k.isoformat()] = v

json.dumps(new_D)

Which returns '{"2010-09-15T23:24:36.169710": "foo", "2010-09-15T23:24:36.169723": "bar"}'. For niceties, wrap it in a function :-)