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Accustom is a library for responding to Custom Resources in AWS CloudFormation using the decorator pattern.

Project description

CloudFormation Accustom

CloudFormation Accustom is a library for responding to Custom Resources in AWS CloudFormation using the decorator pattern.

This library provides a cfnresponse method, some helper static classes, and some decorator methods to help with the function.


CloudFormation Accustom can be found under PyPI at

To install:

python3 -m pip install accustom

To create a Lambda Code Bundle in Zip Format with CloudFormation Accustom and dependencies (including requests), create a directory with only your code in it and run the following. Alternatively you can create a Lambda Layer with CloudFormation Accustom and dependencies installed and use that as your base layer for custom resources.

python3 -m pip install accustom -t .
zip * -r


The quickest way to use this library is to use the standalone decorator @accustom.sdecorator, in a Lambda function.

import accustom
def resource_handler(event, context):
     result = (float(event['ResourceProperties']['key1']) +
     return { 'sum' : result }

In this configuration, the decorator will check to make sure the properties key1 and key2 have been passed by the user, and automatically send a response back to CloudFormation based upon the event object.

As you can see, this greatly simplifies the developer effort required to get a working custom resource that will correctly respond to CloudFormation Custom Resource Requests.

The Decorator Patterns

The most important part of this library are the Decorator patterns. These provide Python decorators that can be put around handler functions, or resource specific functions, that prepare the data for ease of usage. These decorators will also handle exceptions for you.


This is the primary decorator in the library. The purpose of this decorator is to take the return value of the handler function, and respond back to CloudFormation based upon the input event automatically.

It takes the following options:

  • enforceUseOfClass (Boolean) : When this is set to True, you must use a ResponseObject. This is implicitly set to true if no Lambda Context is provided.
  • hideResourceDeleteFailure (Boolean) : When this is set to True the function will return SUCCESS even on getting an Exception for Delete requests.
  • redactConfig (accustom.RedactionConfig) : For more details on how this works please see "Redacting Confidential Information From Logs"
  • timeoutFunction (Boolean): Will automatically send a failure signal to CloudFormation before Lambda timeout provided that this function is executed in Lambda.

Without a ResponseObject the decorator will make the following assumptions:

  • if a Lambda Context is not passed, the function will return FAILED
  • if a dictionary is passed back, this will be used for the Data to be returned to CloudFormation and the function will return SUCCESS.
  • if a string is passed back, this will be put in the return attribute Return and the function will return SUCCESS.
  • if None or True is passed back, the function will return SUCCESS
  • if False is passed back, the function will return FAILED


This decorator, known as the "Resource Decorator" is used when you break the function into different resources, e.g. by making a decision based upon which ResourceType was passed to the handler and calling a function related to that resource.

It takes the following option:

  • decoratorHandleDelete (Boolean) : When set to True, if a Delete request is made in event the decorator will return a ResponseObject with a with SUCCESS without actually executing the decorated function.
  • genUUID (Boolean) : When set to True, if the PhysicalResourceId in the event is not set, automatically generate a UUID4 and put it in the PhysicalResoruceId field.
  • expectedProperties (Array or Tuple) : Pass in a list or tuple of properties that you want to check for before running the decorated function. If any are missing, return FAILED.

The most useful of these options is expectedProperties. With it is possible to quickly define mandatory properties for your resource and fail if they are not included.


This decorator is just a combination of @accustom.decorator() and @accustom.rdecorator(). This allows you to have a single, stand-alone resource handler that has some defined properties and can automatically handle delete. The options available to it is the combination of both of the options available to the other two Decorators, except for redactProperties which takes an accustom.StandaloneRedactionConfig object instead of an accustom.RedactionConfig object. For more information on redactProperties see "Redacting Confidential Information From Logs".

The other important note about combining these two decorators is that hideResourceDeleteFailure becomes redundant if decoratorHandleDelete is set to True.

Response Function and Object

The cfnresponse() function and the ResponseObject are convenience function for interacting with CloudFormation.


cfnresponse() is a traditional function. At the very minimum it needs to take in the event and a status, SUCCESS or FAILED. In practice this function will likely not be used very often outside the library, but it is included for completeness. For more details look directly at the source code for this function.


The ResponseObject allows you to define a message to be sent to CloudFormation. It only has one method, send(), which uses the cfnresponse() function under the hood to fire the event. A response object can be initialised and fired with:

import accustom

def handler(event, context):
    r = accustom.ResponseObject()

If you are using the decorator pattern it is strongly recommended that you do not invoke the send() method, and instead allow the decorator to process the sending of the events for you by returning from your function.

To construct a response object you can provide the following optional parameters:

  • data (Dictionary) : data to be passed in the response. Must be a dict if used
  • physicalResourceId (String) : Physical resource ID to be used in the response
  • reason (String) : Reason to pass back to CloudFormation in the response Object
  • responseStatus (accustom.Status): response Status to use in the response Object, defaults to SUCCESS
  • squashPrintResponse (Boolean) : In DEBUG logging the function will often print out the Data section of the response. If the Data contains confidential information you'll want to squash this output. This option, when set to True, will squash the output.

Logging Recommendations

The decorators utilise the logging library for logging. It is strongly recommended that your function does the same, and sets the logging level to at least INFO. Ensure the log level is set before importing Accustom.

import logging
logger = logging.getLogger(__name__)
import accustom

Redacting Confidential Information From DEBUG Logs

If you often pass confidential information like passwords and secrets in properties to Custom Resources, you may want to prevent certain properties from being printed to debug logs. To help with this we provide a functionality to either blocklist or allowlist Resource Properties based upon provided regular expressions.

To utilise this functionality you must initialise and include a RedactionConfig. A RedactionConfig consists of some flags to define the redaction mode and if the response URL should be redacted, as well as a series of RedactionRuleSet objects that define what to redact based upon regular expressions. There is a special case of RedactionConfig called a StandaloneRedactionConfig that has one, and only one, RedactionRuleSet that is provided at initialisation.

Each RedactionRuleSet defines a single regex that defines which ResourceTypes this rule set should be applied too. You can then apply any number of rules, based upon an explicit property name, or a regex. Please see the definitions, and an example below.


The RedactionRuleSet object allows you to define a series of properties or regexes which to allowlist or blocklist for a given resource type regex. It is initialised with the following:

  • resourceRegex (String) : The regex used to work out what resources to apply this too.


  • propertiesRegex (String) : The regex used to work out what properties to allowlist/blocklist


  • propertyName (String) : The name of the property to allowlist/blocklist


The RedactionConfig object allows you to create a collection of RedactionRuleSet objects as well as define what mode (allowlist/blocklist) to use, and if the presigned URL provided by CloudFormation should be redacted from the logs.

  • redactMode (accustom.RedactMode) : What redaction mode should be used, if it should be a blocklist or allowlist
  • redactResponseURL (Boolean) : If the response URL should be not be logged.


  • ruleSet (accustom.RedactionRuleSet) : The rule set to be added to the RedactionConfig


The StandaloneRedactionConfig object allows you to apply a single RedactionRuleSet object as well as define what mode (allowlist/blocklist) to use, and if the presigned URL provided by CloudFormation should be redacted from the logs.

  • redactMode (accustom.RedactMode) : What redaction mode should be used, if it should be a blocklist or allowlist
  • redactResponseURL (Boolean) : If the response URL should be not be logged.
  • ruleSet (accustom.RedactionRuleSet) : The rule set to be added to the RedactionConfig

Example of Redaction

The below example takes in two rule sets. The first ruleset applies to all resources types, and the second ruleset applies only to the Custom::Test resource type.

All resources will have properties called Test and Example redacted and replaced with [REDATED]. The Custom::Test resource will also additionally redact properties called Custom and those that start with DeleteMe.

Finally, as redactResponseURL is set to True, the response URL will not be printed in the debug logs.

from accustom import RedactionRuleSet, RedactionConfig, decorator

ruleSetDefault = RedactionRuleSet()

ruleSetCustom = RedactionRuleSet('^Custom::Test$')

rc = RedactionConfig(redactResponseURL=True)

def resource_handler(event, context):
    result = (float(event['ResourceProperties']['Test']) +
    return { 'sum' : result }

Note on Timeouts and Permissions

The timeout is implemented using a synchronous chained invocation of your Lambda function. For this reason, please be aware of the following limitations:

  • The function must have access to the Lambda API Endpoints in order to self invoke.
  • The function must have permission to self invoke (i.e. lambda:InvokeFunction permission).

If your requirements violate any of these conditions, set the timeoutFunction option to False. Please also note that this will double the invocations per request, so if you're not in the free tier for Lambda make sure you are aware of this as it may increase costs.


We provide three constants for ease of use:

  • Static value : how to access


  • SUCCESS : accustom.Status.SUCCESS
  • FAILED : accustom.Status.FAILED


  • Create : accustom.RequestType.CREATE
  • Update : accustom.RequestType.UPDATE
  • Delete : accustom.RequestType.DELETE


  • Blocklisting : accustom.RedactMode.BLOCKLIST
  • Allowlisting : accustom.RedactMode.ALLOWLIST

How to Contribute

Feel free to open issues, fork, or submit a pull request:

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