cfn-handler 1.0.0

Modern CloudFormation Custom Resource lifecycle handler for AWS Lambda

cfn-handler

A modern, well-engineered Python library for writing AWS CloudFormation Custom Resource lifecycle handlers. Inspired by — but not derived from — the unmaintained aws-cloudformation/custom-resource-helper.

Why cfn-handler?

CloudFormation Custom Resources are AWS Lambda functions invoked during stack operations. Getting them right is harder than it looks: you must respond to a presigned URL within the Lambda timeout, handle CREATE/UPDATE/DELETE semantics correctly, manage long-running operations via polling, and never leave CloudFormation hanging.

cfn-handler does the boilerplate so you can focus on the resource logic.

from cfn_handler import CustomResource

resource = CustomResource()

@resource.create
def on_create(event, context):
    # do work, return data dict
    return {"Endpoint": "https://my.example.com"}

@resource.update
def on_update(event, context):
    return {"Endpoint": "https://my.example.com"}

@resource.delete
def on_delete(event, context):
    pass

def handler(event, context):
    return resource(event, context)

That's the entire happy path. For long-running operations:

@resource.create
def on_create(event, context):
    # kick off work; return None to defer
    pass

@resource.poll_create
def on_poll_create(event, context):
    # check status; return data when done, raise on failure
    if check_ready():
        return {"Endpoint": "https://my.example.com"}
    # else: do nothing, library will reschedule

Installation

pip install cfn-handler
# or with uv
uv add cfn-handler

cfn-handler requires Python 3.10+ and has zero runtime dependencies. Polling support uses boto3 lazily; boto3 ships preinstalled in the AWS Lambda Python runtimes, so no extra install is needed there.

Comparison to crhelper

If you're coming from crhelper, the model will feel familiar:

Feature	crhelper	cfn-handler
Lifecycle decorators	@helper.create, @helper.update, @helper.delete	@resource.create, @resource.update, @resource.delete
Polling decorators	@helper.poll_create ...	@resource.poll_create ...
Type hints	None	Full inline (py.typed)
Python versions	3.6+ (last release 2020)	3.10–3.14
Build	setup.py	pyproject.toml (PEP 621, hatchling)
Tests	unittest	pytest + hypothesis
Coverage gate	None	95% line + branch
Type checkers	None	mypy strict + pyright strict
Releases	Manual	release-please + PyPI Trusted Publishing
Maintained	No (since 2020)	Yes

The public API is intentionally similar — cfn-handler is a clean re-implementation that carries forward the proven semantics and fixes 14 long-standing upstream issues that never merged. See CHANGELOG.md for the full list.

Examples

Working SAM-deployable examples live in examples/:

• examples/basic/ — minimal Create/Update/Delete handler.
• examples/polled/ — long-running operation with polling.
• examples/with-physical-id/ — explicit physical resource id (replacement on update).
• examples/failing/ — handler that fails, demonstrating FAILED-response semantics.

Project status

v1.0.0 — first stable release. Follows Semantic Versioning. The public API surface is exactly what's exported from cfn_handler.__all__; everything under cfn_handler._internal is implementation detail and may change between minor versions.

Contributing

See CONTRIBUTING.md. For contributors who use Nix, a flake.nix provides a reproducible dev shell.

License

Apache License 2.0 — see LICENSE and NOTICE.

This project is inspired by, but does not include code from, aws-cloudformation/custom-resource-helper. Both projects are licensed under Apache 2.0.