Serializing class instance to JSON

Question

I am trying to create a JSON string representation of a class instance and having difficulty. Let's say the class is built like this:

class testclass:
    value1 = "a"
    value2 = "b"

A call to the json.dumps is made like this:

t = testclass()
json.dumps(t)

It is failing and telling me that the testclass is not JSON serializable.

TypeError: <__main__.testclass object at 0x000000000227A400> is not JSON serializable

I have also tried using the pickle module :

t = testclass()
print(pickle.dumps(t, pickle.HIGHEST_PROTOCOL))

And it gives class instance information but not a serialized content of the class instance.

b'\x80\x03c__main__\ntestclass\nq\x00)\x81q\x01}q\x02b.'

What am I doing wrong?

Answer 1

You can specify the default named parameter in the json.dumps() function:

json.dumps(obj, default=lambda x: x.__dict__)

Explanation:

Form the docs (2.7, 3.6):

``default(obj)`` is a function that should return a serializable version
of obj or raise TypeError. The default simply raises TypeError.

(Works on Python 2.7 and Python 3.x)

Note: In this case you need instance variables and not class variables, as the example in the question tries to do. (I am assuming the asker meant class instance to be an object of a class)

I learned this first from @phihag's answer here. Found it to be the simplest and cleanest way to do the job.

Answer 2

Using jsonpickle

import jsonpickle

object = YourClass()
json_object = jsonpickle.encode(object)

Answer 3

The basic problem is that the JSON encoder json.dumps() only knows how to serialize a limited set of object types by default, all built-in types. List here: https://docs.python.org/3.3/library/json.html#encoders-and-decoders

One good solution would be to make your class inherit from JSONEncoder and then implement the JSONEncoder.default() function, and make that function emit the correct JSON for your class.

A simple solution would be to call json.dumps() on the .__dict__ member of that instance. That is a standard Python dict and if your class is simple it will be JSON serializable.

class Foo(object):
    def __init__(self):
        self.x = 1
        self.y = 2

foo = Foo()
s = json.dumps(foo) # raises TypeError with "is not JSON serializable"

s = json.dumps(foo.__dict__) # s set to: {"x":1, "y":2}

The above approach is discussed in this blog posting:

    Serializing arbitrary Python objects to JSON using __dict__

NOTE: I have edited this answer; the original version only discussed the .__dict__ serializing approach.

Answer 4

There are some good answers on how to get started on doing this. But there are some things to keep in mind:

• What if the instance is nested inside a large data structure?
• What if also want the class name?
• What if you want to deserialize the instance?
• What if you're using __slots__ instead of __dict__?
• What if you just don't want to do it yourself?

json-tricks is a library (that I made and others contributed to) which has been able to do this for quite a while. For example:

class MyTestCls:
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

cls_instance = MyTestCls(s='ub', dct={'7': 7})

json = dumps(cls_instance, indent=4)
instance = loads(json)

You'll get your instance back. Here the json looks like this:

{
    "__instance_type__": [
        "json_tricks.test_class",
        "MyTestCls"
    ],
    "attributes": {
        "s": "ub",
        "dct": {
            "7": 7
        }
    }
}

If you like to make your own solution, you might look at the source of json-tricks so as not to forget some special cases (like __slots__).

It also does other types like numpy arrays, datetimes, complex numbers; it also allows for comments.

Answer 5

Here are two simple functions for serialization of any non-sophisticated classes, nothing fancy as explained before.

I use this for configuration type stuff because I can add new members to the classes with no code adjustments.

import json

class SimpleClass:
    def __init__(self, a=None, b=None, c=None):
        self.a = a
        self.b = b
        self.c = c

def serialize_json(instance=None, path=None):
    dt = {}
    dt.update(vars(instance))

    with open(path, "w") as file:
        json.dump(dt, file)

def deserialize_json(cls=None, path=None):
    def read_json(_path):
        with open(_path, "r") as file:
            return json.load(file)

    data = read_json(path)

    instance = object.__new__(cls)

    for key, value in data.items():
        setattr(instance, key, value)

    return instance

# Usage: Create class and serialize under Windows file system.
write_settings = SimpleClass(a=1, b=2, c=3)
serialize_json(write_settings, r"c:\temp\test.json")

# Read back and rehydrate.
read_settings = deserialize_json(SimpleClass, r"c:\temp\test.json")

# results are the same.
print(vars(write_settings))
print(vars(read_settings))

# output:
# {'c': 3, 'b': 2, 'a': 1}
# {'c': 3, 'b': 2, 'a': 1}

Answer 6

There's one way that works great for me that you can try out:

json.dumps() can take an optional parameter default where you can specify a custom serializer function for unknown types, which in my case looks like

def serialize(obj):
    """JSON serializer for objects not serializable by default json code"""

    if isinstance(obj, date):
        serial = obj.isoformat()
        return serial

    if isinstance(obj, time):
        serial = obj.isoformat()
        return serial

    return obj.__dict__

First two ifs are for date and time serialization and then there is a obj.__dict__ returned for any other object.

the final call looks like:

json.dumps(myObj, default=serialize)

It's especially good when you are serializing a collection and you don't want to call __dict__ explicitly for every object. Here it's done for you automatically.

So far worked so good for me, looking forward for your thoughts.