Extending JSONEncoder to call json for serialization?

Question

I'd like to extend the JSONEncoder to call __json__ of an object to convert it to something serializable.

I've tried this:

import json

class Foo(object):

    def __json__(self):
        return dict(a=1, b=2)

class MyJSONEncoder(json.JSONEncoder):
      """
      Use __json__ attr or callable of object for JSON serialization.
      """
      def default(self, o):
          if hasattr(o, '__json__'):
              if callable(o.__json__):
                  o = o.__json__()
              else:
                  o = o.__json__
          return super(MyJSONEncoder, self).default(o)

json.dumps(Foo(), cls=MyJSONEncoder)

And get this:

Traceback (most recent call last):
  File "./foo.py", line 20, in <module>
    json.dumps(Foo(), cls=MyJSONEncoder)
  File "/usr/lib64/python2.7/json/__init__.py", line 250, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "/usr/lib64/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib64/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "./foo.py", line 18, in default
    return super(MyJSONEncoder, self).default(o)
  File "/usr/lib64/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: {'a': 1, 'b': 2} is not JSON serializable

rrauenza · Accepted Answer · 2016-07-07 16:29:10Z

I found my error:

class MyJSONEncoder(DjangoJSONEncoder):                                         
      """                                                                         
      Use __json__ attr or callable of object for JSON serialization.             
      """                                                                         

      def default(self, o):                                                       
          if hasattr(o, '__json__'):                                              
              if callable(o.__json__):                                            
                  return o.__json__() # <----- here                                            
              else:                                                               
                  return o.__json__ # <-------- and here                                              
          return super(MyJSONEncoder, self).default(o)

The initial implementation mistakenly passed the o.__json__ results up the chain to the parent's default(), with the assumption that default() should return a serialized object.. This isn't how it works. It needs to return a serializable object.

The source of json/encoder.py summarizes this well:

To extend this to recognize other objects, subclass and implement a .default() method with another method that returns a serializable object for o if possible, otherwise it should call the superclass implementation (to raise TypeError).

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