Python: Serialize object to json

Question

So i have the following objects:

import os
import sys
import json

class ComplexEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, P):
            return json.dumps(obj.__dict__)
        # Let the base class default method raise the TypeError
        return json.JSONEncoder.default(self, obj)

class P(object):
    def __init__(self):
        self.name = "Unknown"
        self.id = 1
        self.lst = []

    def showName(self):
        print(self.name)
        for item in self.lst:
            item.showName()

    def add(self, p):
        self.lst.append(p)

    def serialize(self):
        return json.dumps(self, cls=ComplexEncoder)

class PFirst(P):
    def __init__(self):
        P.__init__(self)
        self.name = "First"
        self.id = 2

p1 = PFirst()
p1.showName()

p = P()
p.add(p1)
p.showName()

print(p.serialize())

I want to make a json which reprezents the attributes of object p but i receive the following error:

TypeError: <__main__.PFirst object at 0x000002AA287F1438> is not JSON serializable

Can someone please help me?

kennytm · Accepted Answer · 2017-04-08 15:57:11Z

        return json.dumps(obj.__dict__)

You are calling json.dumps with the default encoder again, which surely don't know how to deal with P objects. If you do want to call json.dumps, use:

        return json.dumps(obj.__dict__, cls=type(self))
#                                     ^~~~~~~~~~~~~~~~

However! If the print the result, we see the result is double-encoded as a string:

"{\"name\": \"Unknown\", \"id\": 1, \"lst\": [\"{\\\"name\\\": \\\"First\\\", \\\"id\\\": 2, \\\"lst\\\": []}\"]}"

If you check the ComplexEncoder example, and also the documentation of default(), we will note that it should return a "serializable object", not the serialized string. So you should really write:

class ComplexEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, P):
            return obj.__dict__         # <-----
        return json.JSONEncoder.default(self, obj)

Output:

{"name": "Unknown", "id": 1, "lst": [{"name": "First", "id": 2, "lst": []}]}

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