object not json serializable

Question

While coding in python I'm getting an error: "Object of type myObject is not JSON serializable"

I tried to use some solutions I saw in other questions, but I didn't find many similarities between my code and the code in those questions, so I found it difficult to adapt the solutions in my code.

A part of my script is as follows. The error occurs in the last line of the first block:

import json
import os
from my_object import myObject
for a in A_set:
 if os.path.isfile(filename):
 with open(filename, 'r') as f:
 json_data = json.load(f)
 object_name = myObject.from_json(json_data)
 else:
 object_name = myObject(a)
 object_name.property_A = property_A
 object_name.property_C = []
 object_name.property_C_add_elements(var)
 with open(filename, 'w') as f:
 json.dump(object_name.to_json(), f)

In another python file is defined the class of 'myObject', where is also the method 'to_json()', which I'm calling in the line of code that produces the error.

class myObject:
 def __init__(self, name):
 self.name = name
 self.property_A = property_A
 self.property_B = property_B
 self.property_C = []
 def property_C_add_elements(self, var): 
 self.property_C.append(var)
 def to_json(self):
 return {
 'Name': self.name,
 'property A': self.property_A,
 'property B': self.property_B,
 'property C': self.property_C
 }
 @classmethod
 def from_json(cls, jason_data):
 object_name = myObject(jason_data['property A'])
 property_B = myObject(jason_data['property B'])
 c_list = []
 for var in jason_data['property C']:
 c_list.append(myObject(var))
 object_name.property_B = property_B
 object_name.property_C = c_list
 return object_name

I would be glad if I could get a solution to that problem. Thanks in advance.

• CristiFati: The question lacks some required data according to [SO]: How to create a Minimal, Complete, and Verifiable example (mcve). For example, none of the files should compile: for a in A_set:, self.property_A = property_A. Please correct the question and also add the stacktrace. And edit the question, don't add comments.
• george lal: Well, it's about a long script that is reading data from an excel file and wants to convert it in json form. It's a bit difficult to produce a minimal and complete code, but I'll give a try. However the second block of code is the complete code of my second file. Thanks for responding.

Answer 1

Here's a modified (and working) version of your code.

custom_object.py:

class CustomObject:
 @classmethod
 def from_json(cls, json_data):
 name = json_data["Name"]
 prop_a = json_data["PropertyA"]
 prop_b = json_data["PropertyB"]
 obj = cls(name, prop_a=prop_a, prop_b=prop_b)
 for var in json_data["PropertyC"]:
 obj.add_c_element(var)
 return obj
 def __init__(self, name, prop_a="", prop_b=""):
 self.name = name
 self.prop_a = prop_a
 self.prop_b = prop_a
 self.prop_c = list()
 def add_c_element(self, var):
 self.prop_c.append(var)
 def to_json(self):
 return {
 "Name": self.name,
 "PropertyA": self.prop_a,
 "PropertyB": self.prop_b,
 "PropertyC": self.prop_c,
 }

code.py:

#!/usr/bin/env python3
import sys
import json
import os
from custom_object import CustomObject
def main():
 filename = "./data.json"
 if os.path.isfile(filename):
 print("Attempting to load object from json file...")
 with open(filename, "r") as f:
 json_data = json.load(f)
 try:
 obj = CustomObject.from_json(json_data)
 except Exception as e:
 print(e)
 return
 print("Object: {:}\n Class: {:s}\n Attributes:" .format(obj, obj.__class__.__name__))
 for k, v in getattr(obj, "__dict__", dict()).items():
 print(" {:s}: {:}".format(k, v))
 else:
 print("Creating dummy object and saving to json...")
 obj = CustomObject("Object name", prop_a="object property a", prop_b="object property b")
 obj.add_c_element(1)
 obj.add_c_element("c element 2")
 with open(filename, "w") as f:
 json.dump(obj.to_json(), f)
 print("Done.")
if __name__ == "__main__":
 print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
 main()

Notes:

• Corrected the errors (or added data that was missing)
• Did some renames (class, objects, attributes) for clarity
• Assumed that prop_a, prop_b are strings (not very important)
• The (main) problem was in from_json function (I'm not sure what you tried to do there: why so many myObject instantiations, when there should have been only one). Anyway, what it does now:
  1. Gets name, prop_a, prop_b attributes from json_data (which is a dict)
  2. Constructs the object out of the 3 values from #1.
  3. Read the objects for prop_c, and adds them one by one (if any) to the object (by calling add_c_element)
• Program searches for a file (with json contents):
  • If found, it tries to load the object from it and displays it
  • If not found, it creates a dummy object and dumps it in the file
• This is one (not a very nice) way of doing things. It's meant to require minimum code changes, and it's also for learning purposes. The proper (scalable, general) way would be to extend JSONEncoder, JSONDecoder ([Python 3]: json - JSON encoder and decoder), but I feel that it would be a bit too advanced at this point

Output:

(py35x64_test) e:\Work\Dev\StackOverflow\q053914912>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code.py
Python 3.5.4 (v3.5.4:3f56838, Aug 8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32
Creating dummy object and saving to json...
Done.
(py35x64_test) e:\Work\Dev\StackOverflow\q053914912>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code.py
Python 3.5.4 (v3.5.4:3f56838, Aug 8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32
Attempting to load object from json file...
Object: <custom_object.CustomObject object at 0x00000230B6182C88>
 Class: CustomObject
 Attributes:
 prop_a: object property a
 prop_b: object property a
 name: Object name
 prop_c: [1, 'c element 2']
Done.

• george lal: Thank you very much. The key was finally as you say in the classmethod, and my mistake was that the attributes should take their values from json_data rather than from a new object creation. Actually that was what I wanted. But as the error occurred few lines away from that point, I missed it. About the code of the script I will have a look and I will possibly use it as a part, or as a whole. Thanks again!

Answer 2

from the code you provided i can only speculate what types exactly your property_A, property_B, property_C variables/attributes are but

@classmethod
def from_json(cls, jason_data):
 object_name = myObject(jason_data['property A'])
 property_B = myObject(jason_data['property B'])
 c_list = []
 for var in jason_data['property C']:
 c_list.append(myObject(var))
 object_name.property_B = property_B
 object_name.property_C = c_list
 return object_name

Let's me speculate that your properties are Classinstances which have no "definition" of how they should be serialized into JSON. This is backed up by what you said :

"Object of type myObject is not JSON serializable"

I guess the problem is with the from_json() classmethod, there you should probably do smth. like :

EDITED:

Assuming that the __init__() method of your Class looks as follows

def __init__(self, name, property_A, property_B, property_C):
 self.name = name
 self.property_A = property_A
 self.property_B = property_B
 self.property_C = property_C 
 # property_C should be a list, you may want to add validation method 
 # which checks for correct types if it is preocess cirtical

I suggest the following:

@classmethod
def from_json(cls, jason_data):
 return cls(
 jason_data['name'],
 jason_data['property A'],
 jason_data['property B'],
 jason_data['property C']
 ) 

Furthermore i suggest, if it's possible, that you change your JSON Format, provided in your to_json() method by simply replacing the whitespaces from "property A" to "property_A" (same for other properties). In addition you can change "Name" to lowercase "name".

Why ? Because with this format, and my changed __init__() method you can convert your from_json() to the following utilizing unpacking operation :

@classmethod
def from_json(cls, jason_data):
 return cls(**json_data) 

• george lal: Well, yes. The type of the error made me not to search carefully for the solution in my own code. Even the error was produced in a line that it was obvious that there wasn't any mistake. However not being familiar yet with the concept of the ool, I can make such a mistake. Now the code works properly. Thank you very much for responding, and for the time you spent.
• CristiFati: Unfortunately, the code in the answer is even more incorrect than the one in the question.
• Chgad: If completely accurate: Yes i was quite superficial and hasty in the first version of my answer. I hope i clarified everything in the edit.