JSON encoding Python list of classes

Question

I've read through a few questions that address something similar, but wanted to ask about this.

I have two Python classes, simplified here:

class Service:
    def __init__(self):
        self.ServiceName = None
        self.ServiceExpDate = None

class Provision:
    def __init__(self):
        self.ID = None
        self.Type = None
        self.Services = [] # a list of Service objects

When I go to JSON encode an instance of the Provision class:

jsonProvision = json.dumps(provision.__dict__)

I get the correct output if I don't have any Services, but if it tries to serialize the Service class I get:

TypeError: <common.Service instance at 0x123d7e8> is not JSON serializable

Do I need to write a JSON encoder to handle this directly, or is there a better way to serialize the Service class?

Thanks!

rotoglup · Accepted Answer · 2012-08-02 14:49:18Z

1

You should write an encoder that takes care of your classes, that's the way json module is meant to be used/extended.

Your attempt to encode the __dict__ of your Provision class instances may work now but is really not future proof if you class evolves.

answered Aug 2, 2012 at 14:49

rotoglup

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Sven Marnach · Accepted Answer · 2012-08-03 11:21:43Z

You need to provide a function to encode your custom classes as the default parameter to json.dumps(). Example code for the classes:

import json

class JSONEncodable(object):
    def json(self):
        return vars(self)

class Service(JSONEncodable):
    def __init__(self):
        self.ServiceName = None
        self.ServiceExpDate = None

class Provision(JSONEncodable):
    def __init__(self):
        self.ID = None
        self.Type = None
        self.Services = [] # a list of Service objects

Example usage:

>>> from operator import methodcaller
>>> p = Provision()
>>> p.Services.append(Service())
>>> print json.dumps(p, default=methodcaller("json"))
{"Services": [{"ServiceName": null, "ServiceExpDate": null}], "Type": null, "ID": null}

You could also use default=attrgetter("__dict__") to avoid the need for a json() method on every class, but the above approach is more flexible.

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