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A collection of useful decorators for making AWS Lambda handlers

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A collection of useful decorators for making AWS Lambda handlers

lambda_decorators is a collection of useful decorators for writing Python handlers for AWS Lambda. They allow you to avoid boiler plate for common things such as CORS headers, JSON serialization, etc.

Quick example

# handler.py

from lambda_decorators import json_http_resp, load_json_body

@json_http_resp
@load_json_body
def handler(event, context):
    return {'hello': event['body']['name']}

When deployed to Lambda behind API Gateway and cURL’d:

$ curl -d '{"name": "world"}' https://example.execute-api.us-east-1.amazonaws.com/dev/hello
{"hello": "world"}

Install

If you are using the serverless framework I recommend using serverless-python-requirements

sls plugin install -n serverless-python-requirements
echo lambda-decorators >> requirements.txt

Or if using some other deployment method to AWS Lambda you can just download the entire module because it’s only one file.

curl -O https://raw.githubusercontent.com/dschep/lambda-decorators/master/lambda_decorators.py

Included Decorators:

lambda_decorators includes the following decorators to avoid boilerplate for common usecases when using AWS Lambda with Python.

See each individual decorators for specific usage details and the example for some more use cases. This library is also meant to serve as an example for how to write decorators for use as lambda middleware. See the recipes page for some more niche examples of using decorators as middleware for lambda.

Writing your own

lambda_decorators includes utilities to make building your own decorators easier. The before, after, and on_exception decorators can be applied to your own functions to turn them into decorators for your handlers. For example:

import logging
from lambda_decorators import before

@before
def log_event(event, context):
    logging.debug(event)
    return event, context

@log_event
def handler(event, context):
    return {}

And if you want to make a decorator that provides two or more of before/after/on_exception functionality, you can use LambdaDecorator:

import logging
from lambda_decorators import LambdaDecorator

class log_everything(LambdaDecorator):
    def before(event, context):
        logging.debug(event, context)
        return event, context
    def after(retval):
        logging.debug(retval)
        return retval
    def on_exception(exception):
        logging.debug(exception)
        return {'statusCode': 500}

@log_everything
def handler(event, context):
    return {}

Why

Initially, I was inspired by middy which I like using in JavaScript. So naturally, I thought I’d like to have something similar in Python too. But then as I thought about it more, it seemed that when thinking of functions as the compute unit, when using python, decorators pretty much are middleware! So instead of building a middleware engine and a few middlewares, I just built a few useful decorators and utilities to build them.


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