Easiest way to serialize a simple class object wit

Question

I'm trying to serialize a list of python objects with JSON (using simplejson) and am getting the error that the object "is not JSON serializable".

The class is a simple class having fields that are only integers, strings, and floats, and inherits similar fields from one parent superclass, e.g.:

class ParentClass:
  def __init__(self, foo):
     self.foo = foo

class ChildClass(ParentClass):
  def __init__(self, foo, bar):
     ParentClass.__init__(self, foo)
     self.bar = bar

bar1 = ChildClass(my_foo, my_bar)
bar2 = ChildClass(my_foo, my_bar)
my_list_of_objects = [bar1, bar2]
simplejson.dump(my_list_of_objects, my_filename)

where foo, bar are simple types like I mentioned above. The only tricky thing is that ChildClass sometimes has a field that refers to another object (of a type that is not ParentClass or ChildClass).

What is the easiest way to serialize this as a json object with simplejson? Is it sufficient to make it serializable as a dictionary? Is the best way to simply write a dict method for ChildClass? Finally, does having the field that refer to another object significantly complicate things? If so, I can rewrite my code to only have simple fields in classes (like strings/floats etc.)

thank you.

Answer 1

I have a similar problem but the json.dump function is not called by me. So, to make MyClass JSON serializable without giving a custom encoder to json.dump you have to Monkey patch the json encoder.

First create your encoder in your module my_module:

import json

class JSONEncoder(json.JSONEncoder):
    """To make MyClass JSON serializable you have to Monkey patch the json
    encoder with the following code:
    >>> import json
    >>> import my_module
    >>> json.JSONEncoder.default = my_module.JSONEncoder.default
    """
    def default(self, o):
        """For JSON serialization."""
        if isinstance(o, MyClass):
            return o.__repr__()
        else:
            return super(self,o)

class MyClass:
    def __repr__(self):
        return "my class representation"

Then as it is described in the comment, monkey patch the json encoder:

import json
import my_module
json.JSONEncoder.default = my_module.JSONEncoder.default

Now, even an call of json.dump in an external library (where you cannot change the cls parameter) will work for your my_module.MyClass objects.