AWS Machine Learning error when creating DataSource from S3

I am having an error when trying to automate AWS DataSource creation from S3: I am running a shell script:

#!/bin/bash 
for k in 1 2 3 4 5 
do
        aws machinelearning create-data-source-from-s3 --cli-input-json file://data/cfg/dsrc_training_00$k.json
        aws machinelearning create-data-source-from-s3 --cli-input-json file://data/cfg/dsrc_validate_00$k.json 
done

and here is an example of the json file it references:

{
   "DataSourceId": "Iris_training_00{k}",
   "DataSourceName": "[DS Iris] training 00{k}",
   "DataSpec": {
      "DataLocationS3": "s3://ml-test-predicto-bucket/shuffled_{k}.csv",
      "DataSchemaLocationS3": "s3://ml-test-predicto-bucket/dsrc_iris.csv.schema",
      "DataRearrangement": {"splitting":{"percentBegin" : 0, "percentEnd" : 70}}
   },
   "ComputeStatistics": true
}

But when I run my script from the command line I get the error:

Parameter validation failed:
Invalid type for parameter DataSpec.DataRearrangement, value: {u'splitting': {u'percentEnd': u'100', u'percentBegin': u'70'}}, type: <type 'dict'>, valid types: <type 'basestring'>

Can someone please help, I have looked at the API AWS ML documentation and I think I am doing everything right, but I can't seem to solve this error... many thanks !

The DataRearrangement element expects a JSON String object. You are passing a dictionary object.

Change:

"DataRearrangement": {"splitting":{"percentBegin" : 0, "percentEnd" : 70}}

[to]

"DataRearrangement": "{\"splitting\":{\"percentBegin\":0,\"percentEnd\":70}}"