AWS Compute Blog

Python 3.12 runtime now available in AWS Lambda

This post is written by Jeff Gebhart, Sr. Specialist TAM, Serverless.

AWS Lambda now supports Python 3.12 as both a managed runtime and container base image. Python 3.12 builds on the performance enhancements that were first released with Python 3.11, and adds a number of performance and language readability features in the interpreter. With this release, Python developers can now take advantage of these new features and enhancements when creating serverless applications on AWS Lambda.

You can use Python 3.12 with Powertools for AWS Lambda (Python), a developer toolkit to implement Serverless best practices such as observability, batch processing, Parameter Store integration, idempotency, feature flags, CloudWatch Metrics, and structured logging among other features.

You can also use Python 3.12 with Lambda@Edge, allowing you to customize low-latency content delivered through Amazon CloudFront.

Python is a popular language for building serverless applications. The Python 3.12 release has a number of interpreter and syntactic improvements.

At launch, new Lambda runtimes receive less usage than existing, established runtimes. This can result in longer cold start times due to reduced cache residency within internal Lambda sub-systems. Cold start times typically improve in the weeks following launch as usage increases. As a result, AWS recommends not drawing conclusions from side-by-side performance comparisons with other Lambda runtimes until the performance has stabilized. Since performance is highly dependent on workload, customers with performance-sensitive workloads should conduct their own testing, instead of relying on generic test benchmarks.

Lambda runtime changes

Amazon Linux 2023

The Python 3.12 runtime is based on the provided.al2023 runtime, which is based on the Amazon Linux 2023 minimal container image. This OS update brings several improvements over the Amazon Linux 2 (AL2)-based OS used for Lambda Python runtimes from Python 3.8 to Python 3.11.

provided.al2023 contains only the essential components necessary to install other packages and offers a smaller deployment footprint of less than 40MB compared to over 100MB for Lambda’s AL2-based images.

With glibc version 2.34, customers have access to a modern version of glibc, updated from version 2.26 in AL2-based images.

The Amazon Linux 2023 minimal image uses microdnf as a package manager, symlinked as dnf. This replaces the yum package manager used in earlier AL2-based images. If you deploy your Lambda functions as container images, you must update your Dockerfiles to use dnf instead of yum when upgrading to the Python 3.12 base image.

Additionally, curl and gnupg2 are also included as their minimal versions curl-minimal and gnupg2-minimal.

Learn more about the provided.al2023 runtime in the blog post Introducing the Amazon Linux 2023 runtime for AWS Lambda and the Amazon Linux 2023 launch blog post.

Response format change

Starting with the Python 3.12 runtime, functions return Unicode characters as part of their JSON response. Previous versions return escaped sequences for Unicode characters in responses.

For example, in Python 3.11, if you return a Unicode string such as “こんにちは”, it escapes the Unicode characters and returns “\u3053\u3093\u306b\u3061\u306f”. The Python 3.12 runtime returns the original “こんにちは”.

This change reduces the size of the payload returned by Lambda. In the previous example, the escaped version is 32 bytes compared to 17 bytes with the Unicode string. Using Unicode responses reduces the size of Lambda responses, making it easier to fit larger responses into the 6MB Lambda response (synchronous) limit.

When upgrading to Python 3.12, you may need to adjust your code in other modules to account for this new behavior. If the caller expects escaped Unicode based on the previous runtime behavior, you must either add code to the returning function to escape the Unicode manually, or adjust the caller to handle the Unicode return.

Extensions processing for graceful shutdown

Lambda functions with external extensions can now benefit from improved graceful shutdown capabilities. When the Lambda service is about to shut down the runtime, it sends a SIGTERM signal to the runtime and then a SHUTDOWN event to each registered external extension.

These events are sent each time an execution environment shuts down. This allows you to catch the SIGTERM signal in your Lambda function and clean up resources, such as database connections, which were created by the function.

To learn more about the Lambda execution environment lifecycle, see Lambda execution environment. More details and examples of how to use graceful shutdown with extensions is available in the AWS Samples GitHub repository.

New Python features

Comprehension inlining

With the implementation of PEP 709, dictionary, list, and set comprehensions are now inlined. Prior versions create a single-use function to execute such comprehensions. Removing this overhead results in faster comprehension execution by a factor of two.

There are some behavior changes to comprehensions because of this update. For example, a call to the ‘locals()’ function from within the comprehension now includes objects from the containing scope, not just within the comprehension itself as in prior versions. You should test functions you are migrating from an earlier version of Python to Python 3.12.

Typing changes

Python 3.12 continues the evolution of including type annotations to Python. PEP 695 includes a new, more compact syntax for generic classes and functions, and adds a new “type” statement to allow for type alias creation. Type aliases are evaluated on demand. This permits aliases to refer to other types defined later.

Type parameters are visible within the scope of the declaration and any nested scopes, but not in the outer scope.

Formalization of f-strings

One of the largest changes in Python 3.12, the formalization of f-strings syntax, is covered under PEP 701. Any valid expression can now be contained within an f-string, including other f-strings.

In prior versions of Python, reusing quotes within an f-string results in errors. With Python 3.12, quote reuse is fully supported in nested f-strings such as the following example:

>>>songs = ['Take me back to Eden', 'Alkaline', 'Ascensionism']

>>>f"This is the playlist: {", ".join(songs)}"

'This is the playlist: Take me back to Eden, Alkaline, Ascensionism'

Additionally, any valid Python expression can be contained within an f-string. This includes multi-line expressions and the ability to embed comments within an f-string.

Before Python 3.12, the “\” character is not permitted within an f-string. This prevented use of “\N” syntax for defining escaped Unicode characters within the body of an f-string.

Asyncio improvements

There are a number of improvements to the asyncio module. These include performance improvements to writing of sockets and a new implementation of asyncio.current_task() that can yield a 4–6 times performance improvement. Event loops now optimize their child watchers for their underlying environment.

Using Python 3.12 in Lambda

AWS Management Console

To use the Python 3.12 runtime to develop your Lambda functions, specify a runtime parameter value Python 3.12 when creating or updating a function. The Python 3.12 version is now available in the Runtime dropdown in the Create Function page:

To update an existing Lambda function to Python 3.12, navigate to the function in the Lambda console and choose Edit in the Runtime settings panel. The new version of Python is available in the Runtime dropdown:

AWS Lambda container image

Change the Python base image version by modifying the FROM statement in your Dockerfile:

# Copy function code

Customers running the Python 3.12 Docker images locally, including customers using AWS SAM, must upgrade their Docker install to version 20.10.10 or later.

AWS Serverless Application Model (AWS SAM)

In AWS SAM set the Runtime attribute to python3.12 to use this version.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: Simple Lambda Function
    Type: AWS::Serverless::Function
      Description: My Python Lambda Function
      CodeUri: my_function/
      Handler: lambda_function.lambda_handler
      Runtime: python3.12

AWS SAM supports generating this template with Python 3.12 for new serverless applications using the `sam init` command. Refer to the AWS SAM documentation.

AWS Cloud Development Kit (AWS CDK)

In AWS CDK, set the runtime attribute to Runtime.PYTHON_3_12 to use this version. In Python CDK:

from constructs import Construct 
from aws_cdk import ( App, Stack, aws_lambda as _lambda )

class SampleLambdaStack(Stack):
    def __init__(self, scope: Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)
        base_lambda = _lambda.Function(self, 'SampleLambda', 

In TypeScript CDK:

import * as cdk from 'aws-cdk-lib';
import * as lambda from 'aws-cdk-lib/aws-lambda'
import * as path from 'path';
import { Construct } from 'constructs';

export class CdkStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // The code that defines your stack goes here

    // The python3.12 enabled Lambda Function
    const lambdaFunction = new lambda.Function(this, 'python311LambdaFunction', {
      runtime: lambda.Runtime.PYTHON_3_12,
      memorySize: 512,
      code: lambda.Code.fromAsset(path.join(__dirname, '/../lambda')),
      handler: 'lambda_handler.handler'


Lambda now supports Python 3.12. This release uses the Amazon Linux 2023 OS, supports Unicode responses, and graceful shutdown for functions with external extensions, and Python 3.12 language features.

You can build and deploy functions using Python 3.12 using the AWS Management Console, AWS CLI, AWS SDK, AWS SAM, AWS CDK, or your choice of Infrastructure as Code (IaC) tool. You can also use the Python 3.12 container base image if you prefer to build and deploy your functions using container images.

Python 3.12 runtime support helps developers to build more efficient, powerful, and scalable serverless applications. Try the Python 3.12 runtime in Lambda today and experience the benefits of this updated language version.

For more serverless learning resources, visit Serverless Land.