>>> import json
  >>> json.dumps(set())
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python2.7/json/__init__.py", line 231, in dumps
      return _default_encoder.encode(obj)
    File "/usr/lib/python2.7/json/encoder.py", line 201, in encode
      chunks = self.iterencode(o, _one_shot=True)
    File "/usr/lib/python2.7/json/encoder.py", line 264, in iterencode
      return _iterencode(o, 0)
    File "/usr/lib/python2.7/json/encoder.py", line 178, in default
      raise TypeError(repr(o) + " is not JSON serializable")
  TypeError: set([]) is not JSON serializable
  >>> from decimal import Decimal
  >>> json.dumps(Decimal(1))
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python2.7/json/__init__.py", line 231, in dumps
      return _default_encoder.encode(obj)
    File "/usr/lib/python2.7/json/encoder.py", line 201, in encode
      chunks = self.iterencode(o, _one_shot=True)
    File "/usr/lib/python2.7/json/encoder.py", line 264, in iterencode
      return _iterencode(o, 0)
    File "/usr/lib/python2.7/json/encoder.py", line 178, in default
      raise TypeError(repr(o) + " is not JSON serializable")
  TypeError: Decimal('1') is not JSON serializable