Working with JSON is an ever present task.

While the builtin json module in Python handles serialization for "standard" data types we regulary encounter common types that are not json serializable. (i.e. datetime, Decimal and more).

These "non-standard" types result in a TypeError. To solve this error, I often see programers looping through a series of if statements.

There is a better way! That way is extending the builtin JSONEncoder.

Encoding Standard Python types

No issue here.

import json

standard = {
    "int": 10,
    "string": "iamastring",
    "iterable": [1, 2],
}
print(json.dumps(standard))

# Output:
# {"int": 10, "string": "iamastring", "iterable": [1, 2]}

O no! Encoding Non-standard types

We have an issue.

from decimal import Decimal
from datetime import datetime

non_standard = {
    "datetime": datetime.utcnow(),
    "decimal": Decimal(10.5),
}
print(json.dumps(non_standard))

# Output:
# TypeError: Object of type datetime is not JSON serializable

Solution

Extend the JSONEncoder class to make handling non-standard types quick, easy and carefree.

from json import JSONEncoder

class CustomJsonEncoder(JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        if isinstance(obj, Decimal):
            return float(obj)
        else:
            return JSONEncoder.default(self, obj)

print(json.dumps(non_standard, cls=CustomJsonEncoder))

# Output:
# {"datettime": "2023-12-25T17:38:32.872132", "decimal": 10.5}

By specifying a CustomerJsonEncoder we can serialize without worry. This pattern can be extended to handle any other non-standard type we may desire.