How to make Marshmallow (>=3.0) treat numbers in fields.Dict() as Decimals

Question

I have a JSON API, that has one of the fields defined as fields.Dict - the data can contain valid JSON and it's content is left to User to decide.

However, there now is a problem that the user cannot use Decimals (need to store them as strings) as the storage backend (DynamoDB) only supports Decimal not float - and passing json.loads(... parse_float=Decimal) does not work as the deserialization is done by Marshmallow.

How can I either force marshmallow to (recursively) treat all numbers with . in them in fields.Dict as Decimals or force marshmallow to always pass the parse_float parameter to JSON load?

Answer 1

How can I [...] force marshmallow to always pass the parse_float parameter to JSON load?

You can specify a custom render_module.

import json

class MyJsonDecimalLoader:
    @staticmethod
    def loads(data):
        return json.loads(data, parse_float=Decimal)

class MySchema(marshmallow.Schema):
    class Meta:
        render_module = MyJsonDecimalLoader

    [...]

I don't see how to do better just using marshmallow fields since the data structure in the dict values is loosely defined.