Method to serialize custom object to JSON in Python [duplicate]

Question

Generally, we can define __str__ method to make str(obj) return what we want.

But now I want to define my Model object to return a default JSON string when using json.dumps(obj).

Is there any nice way for me to declare a method in the class to do this?

class MyClass:
    ...
    def __json__(self):
        return {'name': self.name, 'age': self.age}

obj = MyClass()

json.dumps(obj) # returns the same as json.dumps(obj.__json__)

I don't concern about that, loads returns dict is ok. @user2357112 — Alfred Huang
– Alfred Huang, Commented Jun 4, 2014 at 6:19
The python json module docs includes short examples for extending to custom classes. Their example is complex numbers. Could you try adapting that? — Paul
– Paul, Commented Jun 4, 2014 at 6:22

otus · Accepted Answer · 2014-06-04 07:12:51Z

9

If you only need Python -> JSON, it's simple to write a JSONEncoder class that calls such a method for any object:

class AutoJSONEncoder(JSONEncoder):
    def default(self, obj):
        try:
            return obj._json()
        except AttributeError:
            return JSONEncoder.default(self, obj)

You can then use the class directly AutoJSONEncoder().encode(obj) or through the dumps interface json.dumps(obj, cls=AutoJSONEncoder).

The reverse direction requires at least a list of classes for which to call a _fromjson method.

(Note: __foo__ names are reserved so you shouldn't define them for your own purposes. __bar invokes name mangling, which probably isn't what you want.)

edited Jun 4, 2014 at 7:12

answered Jun 4, 2014 at 6:33

otus

5,7421 gold badge38 silver badges51 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Alfred Huang Over a year ago

Well, this is I want. But I tried the code, it seems not works by default, how can it be?

otus Over a year ago

You need to make sure you are using the custom class instead of the default JSONEncoder. You can pass the class name to json.dumps using the cls parameter. If you already have a bunch of dumps calls and don't want to add it to all of them, you can define your own using dumps = lambda obj: json.dumps(obj, cls=AutoJSONEncoder).

Alfred Huang Over a year ago

OK, thanks alot! I edited your answer to add the call method.

Alfred Huang Over a year ago

For more, when I tried AutoJSONEncoder().encode([obj1, obj2]), I got an error, can this be possible?

Alfred Huang Over a year ago

It raises [<...>] is not JSON serializable, Exception Type: TypeError, I just want it automatically cascading, is that possible? @otus

|

Jakub Roztocil · Accepted Answer · 2014-06-04 06:33:44Z

9

import json


class MyEncoder(json.JSONEncoder):
    """
    JSONEncoder subclass that leverages an object's `__json__()` method,
    if available, to obtain its default JSON representation. 

    """
    def default(self, obj):
        if hasattr(obj, '__json__'):
            return obj.__json__()
        return json.JSONEncoder.default(self, obj)


class MyClass(object):
    name = 'John'
    age = 30

    def __json__(self):
        return {'name': self.name, 'age': self.age}

>>> json.dumps(MyClass(), cls=MyEncoder)
{"age": 30, "name": "John"}

answered Jun 4, 2014 at 6:33

Jakub Roztocil

16.3k5 gold badges53 silver badges52 bronze badges

Collectives™ on Stack Overflow

Method to serialize custom object to JSON in Python [duplicate]

2 Answers 2

6 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Linked

Related