Python json TypeError: Object of type Decimal is not JSON serializable

Question

I did check a lot of d iffe rent rel ate d Q'n'A (see linked questions), but none adressed my specific problem, mostly it was about different datatypes or requirements.

The attempt is to use this snippet to convert a nested map to json:

import json
#...
result = json.dumps(table_scan)

I am aware of the problems of this working correctly, but this doesnt seem to be the problem here. The above unfortunately gives me:

[ERROR] TypeError: Object of type Decimal is not JSON serializable
Traceback (most recent call last):
  File "/var/task/lambda-function.py", line 61, in lambda_handler
    result
  File "/var/lang/lib/python3.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/var/lang/lib/python3.7/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/var/lang/lib/python3.7/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/var/lang/lib/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '

How can I fix this without having to go into the (quite large) map for a 'manual' conversion.

(Application context more precisely: My python AWS lambda function performs scanning a DynamoDB with boto3 which gives me a (multi-layer) dict≈map. My goal is to convert this map to a json in order to return it (outwards) to a AWS API gateway. The items inside the Database come from the event parameters of the (inwards) API gateway.)

Cadoiz · Accepted Answer · 2021-09-14 06:44:33Z

4

A custom encoder for specific json values (including Decimal) can be used as suggested here. In this example, Decimal should presumably be converted to float:

import json
from decimal import Decimal

#Does quasi the same things as json.loads from here: https://pypi.org/project/dynamodb-json/
class JSONEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, Decimal):
            return float(obj)
        return json.JSONEncoder.default(self, obj)

#...
result = json.dumps(table_scan, cls=JSONEncoder)  #Works!

edited Sep 14, 2021 at 6:44

answered Sep 8, 2021 at 14:01

Cadoiz

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1 Comment

macnewbold Over a year ago

Note that as others have pointed out so well on stackoverflow.com/questions/43678946/… there are inaccuracies that can arise from converting a decimal to a float. They suggest using str() rather than float() to avoid losing any precision or creating any rounding errors, which was presumably a reason for using Decimal in the first place.

i_want_more_edits · Accepted Answer · 2021-09-13 09:05:37Z

Unfortunately, I'm not yet able to post this as a comment.

You can consider monkey-patching the missing feature instead of writing your own JSONEncoder. To do so, you can (for the sake of simplicity) write an own small standalone module, say make_json_serializable.py to apply this monkey patch (what is it?) to the

json module when it's imported so JSONEncoder.default() automatically checks for a special "to_json()" method and uses it to encode the object if found. (Quote from here)

Code:

from json import JSONEncoder

def _default(self, obj):
    return getattr(obj.__class__, "to_json", _default.default)(obj)

_default.default = JSONEncoder.default  # Save unmodified default.
JSONEncoder.default = _default # Replace it.

Then you can add an to_json method to any class like taken from this answer:

def to_json(self):
  return float(self) # or how you want it to be serialized

Then you also don't have to update every json.dumps() in your project like Onur said here. Applying this monkey-patch should work like this (untested):

from decimal import Decimal
Decimal.to_json = to_json

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