AWS Python Lambda: how to raise Exception for step function

Question

I have the following flow for my AWS step function, how should my Python lambda raise MyCustomError?

Just use raise Exception("MyCustomError")? Or I need to do something else? The official doc at https://docs.aws.amazon.com/step-functions/latest/dg/concepts-error-handling.html use node.js as example, and I don't see any Python examples.

{ "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function", "StartAt": "HelloWorld", "States": { "HelloWorld": { "Type": "Task", "Resource": "arn:aws:lambda:us-east-1:123456789012:function:FailFunction", "Retry": [ { "ErrorEquals": ["MyCustomError"], "IntervalSeconds": 1, "MaxAttempts": 2, "BackoffRate": 2.0 } ], "End": true } } }

At a guess, define MyCustomError as a subclass of Exception and then raise MyCustomError(). — John Gordon
– John Gordon, Commented Apr 19, 2021 at 17:35

Grant Lammi · Accepted Answer · 2021-04-19 20:51:16Z

I did something super similar to this when I needed to catch and retry an API call we made. On the first connection to Aurora Serverless it can take 30 seconds or so to spin up the cluster. So if we got a timeout I just wanted to throw an exception that Step Functions would then retry.

The Step Function state looks like this, with a different wait for my custom exception versus the standard Lambda ones:

"Hello World": { "Type": "Task", "Resource": "arn:aws:lambda:us-east-1:123456789012:function:FailFunction" "Retry": [ { "ErrorEquals": [ "Lambda.ServiceException", "Lambda.AWSLambdaException", "Lambda.SdkClientException" ], "IntervalSeconds": 2, "MaxAttempts": 6, "BackoffRate": 2 }, { "ErrorEquals": [ "QueryAPIUnavailableException" ], "IntervalSeconds": 30, "MaxAttempts": 5, "BackoffRate": 2 } ], "End": true }

And then the Lambda itself just does a raise on an Exception subclass that is nothing but a pass:

class QueryAPIUnavailableException(Exception): pass def lambda_handler(event, context): message = my_query_api.get_message() if (message == 'Endpoint request timed out'): logger.info("Query API timed out, throwing exception for Step Function retry") raise QueryAPIUnavailableException(message) else: print(f"Got back message: {message}")

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