How to make a class JSON serializable

Question

How to make a Python class serializable?

A simple class:

class FileItem:
    def __init__(self, fname):
        self.fname = fname

What should I do to be able to get output of:

json.dumps()

Without an error (FileItem instance at ... is not JSON serializable)

Answer 1

This is a small library that serializes an object with all its children to JSON and also parses it back:

https://github.com/Toubs/PyJSONSerialization/

Answer 2

I like Onur's answer but would expand to include an optional toJSON() method for objects to serialize themselves:

def dumper(obj):
    try:
        return obj.toJSON()
    except:
        return obj.__dict__
print json.dumps(some_big_object, default=dumper, indent=2)

Answer 3

There are many approaches to this problem. 'ObjDict' (pip install objdict) is another. There is an emphasis on providing javascript like objects which can also act like dictionaries to best handle data loaded from JSON, but there are other features which can be useful as well. This provides another alternative solution to the original problem.

Answer 4

import json

class Foo(object):
    def __init__(self):
        self.bar = 'baz'
        self._qux = 'flub'

    def somemethod(self):
        pass

def default(instance):
    return {k: v
            for k, v in vars(instance).items()
            if not str(k).startswith('_')}

json_foo = json.dumps(Foo(), default=default)
assert '{"bar": "baz"}' == json_foo

print(json_foo)

Answer 5

If you don't mind installing a package for it, you can use json-tricks:

pip install json-tricks

After that you just need to import dump(s) from json_tricks instead of json, and it'll usually work:

from json_tricks import dumps
json_str = dumps(cls_instance, indent=4)

which'll give

{
        "__instance_type__": [
                "module_name.test_class",
                "MyTestCls"
        ],
        "attributes": {
                "attr": "val",
                "dct_attr": {
                        "hello": 42
                }
        }
}

And that's basically it!

---

This will work great in general. There are some exceptions, e.g. if special things happen in __new__, or more metaclass magic is going on.

Obviously loading also works (otherwise what's the point):

from json_tricks import loads
json_str = loads(json_str)

This does assume that module_name.test_class.MyTestCls can be imported and hasn't changed in non-compatible ways. You'll get back an instance, not some dictionary or something, and it should be an identical copy to the one you dumped.

If you want to customize how something gets (de)serialized, you can add special methods to your class, like so:

class CustomEncodeCls:
        def __init__(self):
                self.relevant = 42
                self.irrelevant = 37

        def __json_encode__(self):
                # should return primitive, serializable types like dict, list, int, string, float...
                return {'relevant': self.relevant}

        def __json_decode__(self, **attrs):
                # should initialize all properties; note that __init__ is not called implicitly
                self.relevant = attrs['relevant']
                self.irrelevant = 12

which serializes only part of the attributes parameters, as an example.

And as a free bonus, you get (de)serialization of numpy arrays, date & times, ordered maps, as well as the ability to include comments in json.

Disclaimer: I created json_tricks, because I had the same problem as you.

Answer 6

jaraco gave a pretty neat answer. I needed to fix some minor things, but this works:

Code

# Your custom class
class MyCustom(object):
    def __json__(self):
        return {
            'a': self.a,
            'b': self.b,
            '__python__': 'mymodule.submodule:MyCustom.from_json',
        }

    to_json = __json__  # supported by simplejson

    @classmethod
    def from_json(cls, json):
        obj = cls()
        obj.a = json['a']
        obj.b = json['b']
        return obj

# Dumping and loading
import simplejson

obj = MyCustom()
obj.a = 3
obj.b = 4

json = simplejson.dumps(obj, for_json=True)

# Two-step loading
obj2_dict = simplejson.loads(json)
obj2 = MyCustom.from_json(obj2_dict)

# Make sure we have the correct thing
assert isinstance(obj2, MyCustom)
assert obj2.__dict__ == obj.__dict__

Note that we need two steps for loading. For now, the __python__ property is not used.

How common is this?

Using the method of AlJohri, I check popularity of approaches:

Serialization (Python -> JSON):

• to_json: 266,595 on 2018-06-27
• toJSON: 96,307 on 2018-06-27
• __json__: 8,504 on 2018-06-27
• for_json: 6,937 on 2018-06-27

Deserialization (JSON -> Python):

• from_json: 226,101 on 2018-06-27