Implementing custom JSON logic for the Python json module.

custom_serializer_class.py

import json
from json import JSONEncoder

class MyComplexObject(object):
  """
  A trivial Python object that has its own special serialized output
  that for some reason would differ from __repr__.
  """
  
  def __init__(self, arbitrary_value, optional_value=None):
    self.arbitrary_value = arbitrary_value
    self.optional_value = optional_value
    
  def prepared(self):
    if self.optional_value:
      return "{0} ({1})".format(self.arbitrary_value, self.optional_value)
    else:
      return self.arbitrary_value


class MyEncoder(JSONEncoder):
  def default(self, o):
    if isinstance(o, MyComplexObject):
      return o.prepared()
    return JSONEncoder.default(self, o)
    
    
complex_object = MyComplexObject('foo', optional_value='bar')
data = {
  'object': complex_object,
  'number': 2,
  'metadata': {
      'is_hungry': True,
      'banana_count': 0
    }
  }
  
output = json.dumps(data, cls=MyEncoder)
expected = '{"object": "foo (bar)", "number": 2, "metadata": {"is_hungry": true, "banana_count": 0}}'

assert output == expected