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May 2, 2019 ~ 1 min read

Decimals and DynamoDB


A short note on DynamoDB of AWS and how it suprisingly gives Decimals when you extract records. And forced me to write ducktape-code.

The story

DynamoDB implicitely converts integers to decimals. Decimals cause some problems in Python code since it is not a basic type. For example, json.dumps() doesn't work anymore.

  1. You put a Python datastructure with lists and dictionairies into DynamoDB with strings and integers.
  2. DynamoDB converts and stores the datastructure
  3. You load an entry from DynamoDB
  4. ...it suddenly contains Decimals instead of integers
  5. ...and json.dumps() crashes (among others)

The 'solution'

I now simply convert decimals back to float and int. The alternative of storing the object as a string (convert to JSON) I dislike. Figuring out how to configure DynamoDB...I couldn't find a decent way within half an hour. I did find a discussion on a feature request to disable decimals and/or find some sane way to work with it, but no solutions.

So I wrote a little function to convert a dictionary:

def convert_decimal(dictionary):
    """ Converts decimals to float and int. """

    for key in dictionary.keys():
        value = dictionairy[key]
        if isinstance(value, Decimal):
            if v % 1 == 0:
                dictionary[key] = int(value)
            else:
                dictionary[key] = float(value)

    return dictionary

Ruben van der Zwaan

Hi, I'm Ruben. I'm a data scientist / engineer from the Netherlands. I run my own company Datasprong.