Find reasons of failed DynamoDB Transaction with boto3

 When working with Transactions in DynamoDB, you pack several actions into one query. When one of them failed, all the queries are rolled back. But how do you know which one failed? Reading the documentation, you find the following note:

If using Java, DynamoDB lists the cancellation reasons on the CancellationReasons property. This property is not set for other languages. Transaction cancellation reasons are ordered in the order of requested items, if an item has no error it will have NONE code and Null message.

 Why only in Java? I'm working in Python, using the boto3 library. Do I still have a way to access the information? As it turns out, there is a workaround. When I print the message of  my exception, I see the following:

botocore.errorfactory.TransactionCanceledException: An error occurred (TransactionCanceledException) when calling the TransactWriteItems operation: Transaction cancelled, please refer cancellation reasons for specific reasons [None, ConditionalCheckFailed, None]

 The reasons array is printed at the end of the message. If it is already there, why not have it in a more accessible format? From there, it is not difficult to extract it:

import re

def reasons(e):

    m = re.search("\\[(.*)\\]", str(e))

    reasons = m.group(1).split(", ")

    return [None if reason == "None" else reason for reason in reasons]

Now I can use it like that:

import boto3

client = boto3.client("dynamodb")

try:

    client.transact_write_items(

        [

            {

                "ConditionCheck": {... }

            },

            {

                "Put": {

                    "Item": {...},

                    "ConditionExpression": ...

                }

            },

            {

                "Put": {...}

            },

        ]

    )

except client.exceptions.TransactionCanceledException as e:

    if reasons(e)[0] is not None:

        raise MissingItemError()

    raise OperationFailedError()

I hope this array will be added to the exception's response structure as a normal field in a future version. At least, the feature request exists in boto3.