AWS: Find Root Cause of Failure for CloudFormation Stacks

 When a CloudFormation stack fails, you have to scroll back trough the events to find the root cause of the failure. Recently, AWS even added a "Detect Root Cause" button to the Console to immediately scroll to the correct event. But how do you do it from a python script?

import boto3

def find_root_cause(stack_name):

    cf_client = boto3.client('cloudformation')

    next_values = "First Time"

    params = {

        "StackName": stack_name

    }

    root_cause = None

    while next_values:

        result = cf_client.describe_stack_events(**params)

        next_values = result.get("NextToken")

        params["NextToken"] = next_values

        for event in result["StackEvents"]:

            status = event.get("ResourceStatus", "")

            reason = event.get("ResourceStatusReason")

            # start of deployment

            if reason == "User Initiated":

                return root_cause

           

            if reason and "FAILED" in status:

                root_cause = reason

    return root_cause

You follow the same pattern as from the Console. You go back the events history, until you reach the oldest error message before the start of the deployment.