#!/usr/bin/env python

import json as json

data = [ 0.333, 0.999, 0.1 ]
print json.dumps(data)

$ python floatjson.py
[0.33300000000000002, 0.999, 0.10000000000000001]

#!/usr/bin/env python

import json
json.encoder.FLOAT_REPR = lambda x: format(x, '.3f')

data = [ 0.333, 0.999, 0.1 ]
print json.dumps(data)

$ python floatjson.py
[0.333, 0.999, 0.100]