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cfn_resource.py

This project is a decorator and validation system that takes the drudgery out of writing custom resources. You still have access to the context and event as normal, but the decorator handles serializing your response and communicating results to CloudFormation.

See cfn-lambda from Andrew Templeton if you're looking to write your custom resources in Node.js.

Usage

  1. Copy cfn_resource.py into the directory of your lambda function handler.py
  2. Use the cfn_resource.Resource event decorators to decorate your handler like in example.py
  3. Zip up the contents and upload to Lambda

Once the function is up, copy its ARN and use it as the ServiceToken for your custom resource. For more on the requests you may receive, see this document

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Resources": {
        "FakeThing": {
            "Type": "Custom::MyResource",
            "Properties": {
                "ServiceToken": "arn:aws:lambda:SOME-REGION:ACCOUNT:function:FunctionName",
                "OtherThing": "foobar",
                "AnotherThing": 2
            }
        }
    }
}

For more on how custom resources work, see the AWS docs

Code Sample

For this example, you need to have your handler in Lambda set as filename.handler where filename has the below contents.

import cfn_resource

# set `handler` as the entry point for Lambda
handler = cfn_resource.Resource()

@handler.create
def create_thing(event, context):
    # do some stuff
    return {"PhysicalResourceId": "arn:aws:fake:myID"}

@handler.update
def update_thing(event, context):
    # do some stuff
    return {"PhysicalResourceId": "arn:aws:fake:myID"}

Running Tests

To run the tests locally, you need Python 2.7 and pip. Ideally, you should use a virtualenv.

$ pip install -r test-requirements.txt
$ py.test

The tests use mock and py.test and will give you a terminal coverage report. Currently the tests cover ~90% of the (very small) codebase.

License

This code is released under the MIT software license, see LICENSE.txt for details. No warranty of any kind is included, and the copyright notice must be included in redistributions.

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Python decorator for making Lambda-backed CloudFormation resources

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