Extending Python's JSON Encoder

I recently ran into a situation where I needed to ensure consistent formatting of datetimes as I serialized a dict to JSON using python’s built-in json library.

The JSON specification does not specify how to handle dates - so it follows that the JSON encoder does not, by default, include any date formatting.

Attempting to convert a dict which includes a datetime to JSON using the default JSON Encoder will raise a TypeError.

import json
from datetime import datetime

obj = {
    "name": "test",
    "created": datetime.utcnow()
}

obj_default_encoder = json.dumps(obj)
print(obj_default_encoder)
# TypeError: Object of type datetime is not JSON serializable

I was able to achieve the behavior I wanted by defining a class which extends json.JSONEncoder. In this class I included instructions for handling dates - you could also include other modifications to the default behavior of JSONEncoder. Finally, I passed the custom encoder class to the cls kwarg of json.dumps

From the python docs:

To use a custom JSONEncoder subclass (e.g. one that overrides the default() method to serialize additional types), specify it with the cls kwarg; otherwise JSONEncoder is used.

Usage Example

import json
from datetime import datetime

#
# Defining a custom JSON encoder class
#

# Custom class extends json.JSONEncoder
 class CustomJSONEncoder(json.JSONEncoder):

    # Override default() method
    def default(self, obj):

        # Datetime to string
        if isinstance(obj, datetime):
            # Format datetime - `Fri, 21 Aug 2020 17:59:59 GMT`
            obj = obj.strftime('%a, %d %b %Y %H:%M:%S GMT')

            return obj

        # Default behavior for all other types
        return super().default(obj)

#
# Using custom class to serialize a dict to JSON
#

obj = {
    "name": "test",
    "created": datetime.utcnow()
}

obj_custom_encoder = json.dumps(obj, cls=CustomJSONEncoder)
print(obj_custom_encoder)
# {"name": "test", "created": "Fri, 11 Mar 2022 19:11:51 GMT"}