original.json

{
  "Type" : "AWS::ApplicationAutoScaling::ScalingPolicy",
  "Properties" : {
    "PolicyName" : "MyScalingPolicy",
    "PolicyType" : "TargetTrackingScaling",
    "ResourceId" : "arn:aws:dynamodb:us-east-1:123456789012:table/books_table",
    "ScalableDimension" : "dynamodb:table:WriteCapacityUnits",
    "ServiceNamespace" : "dynamodb",
    "TargetTrackingScalingPolicyConfiguration" : {
    "PredefinedMetricSpecification": {
            "PredefinedMetricType": "DynamoDBWriteCapacityUtilization"
        },
        "ScaleOutCooldown": 60,
        "ScaleInCooldown": 60,
        "TargetValue": 50.0
    }    
  }
}

output.json

{
    "Resources": {
        "MyScalingPolicy": {
            "Properties": {
                "PolicyName": "MyScalingPolicy",
                "PolicyType": "TargetTrackingScaling",
                "ResourceId": "arn:aws:dynamodb:us-east-1:123456789012:table/books_table",
                "ScalableDimension": "dynamodb:table:WriteCapacityUnits",
                "ServiceNamespace": "dynamodb",
                "TargetTrackingScalingPolicyConfiguration": {
                    "PredefinedMetricSpecification": {
                        "PredefinedMetricType": "DynamoDBWriteCapacityUtilization"
                    },
                    "ScaleInCooldown": 60,
                    "ScaleOutCooldown": 60,
                    "TargetValue": 50.0
                }
            },
            "Type": "AWS::ApplicationAutoScaling::ScalingPolicy"
        }
    }
}

troposphere_example.py

from troposphere import Template

from troposphere.applicationautoscaling import (
    ScalingPolicy,
    TargetTrackingScalingPolicyConfiguration,
    PredefinedMetricSpecification,
)

table_id = "arn:aws:dynamodb:us-east-1:123456789012:table/books_table"


t = Template()

scaling_policy_config = TargetTrackingScalingPolicyConfiguration(
    PredefinedMetricSpecification=PredefinedMetricSpecification(
        PredefinedMetricType="DynamoDBWriteCapacityUtilization",
    ),
    ScaleOutCooldown=60,
    ScaleInCooldown=60,
    TargetValue=50.0,
)

t.add_resource(
    ScalingPolicy(
        "MyScalingPolicy",
        PolicyName="MyScalingPolicy",
        PolicyType="TargetTrackingScaling",
        ResourceId=table_id,
        ScalableDimension="dynamodb:table:WriteCapacityUnits",
        ServiceNamespace="dynamodb",
        TargetTrackingScalingPolicyConfiguration=scaling_policy_config,
    )
)

print t.to_json()