Today I learned how to do custom JSON encoding in Python with the module json.

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How to encode Python objects as JSON

In Python, we use the module json whenever we need to work with JSON data. This module makes it very easy to do so, by means of the methods json.dump and json.dumps.

By using json.dumps, we can get a JSON document as a string, based on the Python object we give it:

import json

data = {
    "one_key": 73,
    "other_key": [

{"one_key": 73, "other_key": [true, false, null]}

The method json.dump is useful when you have a file you want to write the JSON to.

You can read more about json, json.dump, and json.dumps in the json documentation.

Despite being quite useful, the module json has its limitations, especially ones derived from the JSON standard itself. For example, the JSON standard doesn't tell you how to handle complex numbers, and so the module json doesn't know how to deal with them:

>>> import json
>>> json.dumps(complex(2, 3))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\Python39\lib\json\", line 231, in dumps
    return _default_encoder.encode(obj)
  File "C:\Program Files\Python39\lib\json\", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "C:\Program Files\Python39\lib\json\", line 257, in iterencode
    return _iterencode(o, 0)
  File "C:\Program Files\Python39\lib\json\", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type complex is not JSON serializable

So, what can the module json handle?

Default JSON conversion of Python objects

According to the json docs, these are the types supported by default:

Python JSON
dict object
list, tuple array
str string
int, float, int- & float-derived Enums number
True true
False false
None null

If you want to convert anything that is not on the left column of that table, you need to implement your own custom JSON encoder.

Custom JSON encoding of Python objects

If you want to do custom JSON encoding of Python objects that are not included in the table above, you will need to subclass json.JSONEncoder and you will need to override the method .default.

The method json.JSONEncoder.default is called for each object that the module json doesn't know how to encode by default.

For example, if you want to encode complex numbers in JSON, you could do this:

import json

class JSONComplexEncoder(json.JSONEncoder):
    """JSON encoder that also knows how to encode complex numbers."""

    def default(self, obj):
        if isinstance(obj, complex):
            return {"real": obj.real, "imag": obj.imag}

        return super().default(obj)

The reason we call super().default(obj) if the object provided isn't a complex number is explained in the docs. Essentially, the base implementation is responsible for raising the TypeError when we fail to serialise an object.

Now that we have the class JSONComplexEncoder, we can pass it in to the methods json.dump and json.dumps, so that they can use this custom encoder we defined:

>>> c = complex(2, 3)
>>> json.dumps(c, cls=JSONComplexEncoder)
'{"real": 2.0, "imag": 3.0}'

As we can see, the complex number c was properly converted to JSON.

I wrote a more complete article about defining custom JSON encoders and decoders. You can also read the short articles I wrote prior to that, on decoding JSON and on defining a custom JSON decoder.

That's it for now! Stay tuned and I'll see you around!

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