Amazon Lambda function logging in Python

Open the Log groups page on the CloudWatch console.

Choose the log group for your function (/aws/lambda/your-function-name).

Choose a log stream.

Initialize the application using the Hello World Python template.

sam init --app-template hello-world-powertools-python --name sam-app --package-type Zip --runtime python3.9 --no-tracing

Build the app.

cd sam-app && sam build

Deploy the app.

sam deploy --guided

Follow the on-screen prompts. To accept the default options provided in the interactive experience, press Enter.

Get the URL of the deployed application:

aws cloudformation describe-stacks --stack-name sam-app --query 'Stacks[0].Outputs[?OutputKey==`HelloWorldApi`].OutputValue' --output text

Invoke the API endpoint:

curl GET <URL_FROM_PREVIOUS_STEP>

If successful, you'll see this response:

{"message":"hello world"}

To get the logs for the function, run sam logs. For more information, see Working with logs in the Amazon Serverless Application Model Developer Guide.

sam logs --stack-name sam-app

The log output looks like this:

2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:50.371000 INIT_START Runtime Version: python:3.9.v16    Runtime Version ARN: arn:aws:lambda:us-east-1::runtime:07a48df201798d627f2b950f03bb227aab4a655a1d019c3296406f95937e2525
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.112000 START RequestId: d455cfc4-7704-46df-901b-2a5cce9405be Version: $LATEST
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.114000 {
  "level": "INFO",
  "location": "hello:23",
  "message": "Hello world API - HTTP 200",
  "timestamp": "2023-02-03 14:59:51,113+0000",
  "service": "PowertoolsHelloWorld",
  "cold_start": true,
  "function_name": "sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "function_memory_size": "128",
  "function_arn": "arn:aws:lambda:us-east-1:111122223333:function:sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "function_request_id": "d455cfc4-7704-46df-901b-2a5cce9405be",
  "correlation_id": "e73f8aef-5e07-436e-a30b-63e4b23f0047",
  "xray_trace_id": "1-63dd2166-434a12c22e1307ff2114f299"
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.126000 {
  "_aws": {
    "Timestamp": 1675436391126,
    "CloudWatchMetrics": [
      {
        "Namespace": "Powertools",
        "Dimensions": [
          [
            "function_name",
            "service"
          ]
        ],
        "Metrics": [
          {
            "Name": "ColdStart",
            "Unit": "Count"
          }
        ]
      }
    ]
  },
  "function_name": "sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "service": "PowertoolsHelloWorld",
  "ColdStart": [
    1.0
  ]
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.126000 {
  "_aws": {
    "Timestamp": 1675436391126,
    "CloudWatchMetrics": [
      {
        "Namespace": "Powertools",
        "Dimensions": [
          [
            "service"
          ]
        ],
        "Metrics": [
          {
            "Name": "HelloWorldInvocations",
            "Unit": "Count"
          }
        ]
      }
    ]
  },
  "service": "PowertoolsHelloWorld",
  "HelloWorldInvocations": [
    1.0
  ]
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.128000 END RequestId: d455cfc4-7704-46df-901b-2a5cce9405be
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.128000 REPORT RequestId: d455cfc4-7704-46df-901b-2a5cce9405be    Duration: 16.33 ms    Billed Duration: 17 ms    Memory Size: 128 MB    Max Memory Used: 64 MB    Init Duration: 739.46 ms    
XRAY TraceId: 1-63dd2166-434a12c22e1307ff2114f299    SegmentId: 3c5d18d735a1ced0    Sampled: true

This is a public API endpoint that is accessible over the internet. We recommend that you delete the endpoint after testing.

sam delete

Create a project directory for your new application.

mkdir hello-world
cd hello-world

Initialize the app.

cdk init app --language python

Install the Python dependencies.

pip install -r requirements.txt

Create a directory lambda_function under the root folder.

mkdir lambda_function
cd lambda_function

Create a file app.py and add the following code to the file. This is the code for the Lambda function.

from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.utilities.typing import LambdaContext
from aws_lambda_powertools.logging import correlation_paths
from aws_lambda_powertools import Logger
from aws_lambda_powertools import Tracer
from aws_lambda_powertools import Metrics
from aws_lambda_powertools.metrics import MetricUnit

app = APIGatewayRestResolver()
tracer = Tracer()
logger = Logger()
metrics = Metrics(namespace="PowertoolsSample")

@app.get("/hello")
@tracer.capture_method
def hello():
    # adding custom metrics
    # See: https://docs.powertools.aws.dev/lambda-python/latest/core/metrics/
    metrics.add_metric(name="HelloWorldInvocations", unit=MetricUnit.Count, value=1)

    # structured log
    # See: https://docs.powertools.aws.dev/lambda-python/latest/core/logger/
    logger.info("Hello world API - HTTP 200")
    return {"message": "hello world"}

# Enrich logging with contextual information from Lambda
@logger.inject_lambda_context(correlation_id_path=correlation_paths.API_GATEWAY_REST)
# Adding tracer
# See: https://docs.powertools.aws.dev/lambda-python/latest/core/tracer/
@tracer.capture_lambda_handler
# ensures metrics are flushed upon request completion/failure and capturing ColdStart metric
@metrics.log_metrics(capture_cold_start_metric=True)
def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

Open the hello_world directory. You should see a file called hello_world_stack.py.

cd ..
cd hello_world

Open hello_world_stack.py and add the following code to the file. This contains the Lambda Constructor, which creates the Lambda function, configures environment variables for Powertools and sets log retention to one week, and the ApiGatewayv1 Constructor, which creates the REST API.

from aws_cdk import (
    Stack,
    aws_apigateway as apigwv1,
    aws_lambda as lambda_,
    CfnOutput,
    Duration
)
from constructs import Construct

class HelloWorldStack(Stack):

    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        # Powertools Lambda Layer
        powertools_layer = lambda_.LayerVersion.from_layer_version_arn(
            self,
            id="lambda-powertools",
            # At the moment we wrote this example, the aws_lambda_python_alpha CDK constructor is in Alpha, o we use layer to make the example simpler
            # See https://docs.aws.amazon.com/cdk/api/v2/python/aws_cdk.aws_lambda_python_alpha/README.html
            # Check all Powertools layers versions here: https://docs.powertools.aws.dev/lambda-python/latest/#lambda-layer
            layer_version_arn=f"arn:aws:lambda:{self.region}:017000801446:layer:AWSLambdaPowertoolsPythonV2:21"
        )

        function = lambda_.Function(self,
            'sample-app-lambda',
            runtime=lambda_.Runtime.PYTHON_3_9,
            layers=[powertools_layer],
            code = lambda_.Code.from_asset("./lambda_function/"),
            handler="app.lambda_handler",
            memory_size=128,
            timeout=Duration.seconds(3),
            architecture=lambda_.Architecture.X86_64,
            environment={
                "POWERTOOLS_SERVICE_NAME": "PowertoolsHelloWorld",
                "POWERTOOLS_METRICS_NAMESPACE": "PowertoolsSample",
                "LOG_LEVEL": "INFO"
            }
        )

        apigw = apigwv1.RestApi(self, "PowertoolsAPI", deploy_options=apigwv1.StageOptions(stage_name="dev"))

        hello_api = apigw.root.add_resource("hello")
        hello_api.add_method("GET", apigwv1.LambdaIntegration(function, proxy=True))

        CfnOutput(self, "apiUrl", value=f"{apigw.url}hello")

Deploy your application.

cd ..
cdk deploy

Get the URL of the deployed application:

aws cloudformation describe-stacks --stack-name HelloWorldStack --query 'Stacks[0].Outputs[?OutputKey==`apiUrl`].OutputValue' --output text

Invoke the API endpoint:

curl GET <URL_FROM_PREVIOUS_STEP>

If successful, you'll see this response:

{"message":"hello world"}

To get the logs for the function, run sam logs. For more information, see Working with logs in the Amazon Serverless Application Model Developer Guide.

sam logs --stack-name HelloWorldStack

The log output looks like this:

2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:50.371000 INIT_START Runtime Version: python:3.9.v16    Runtime Version ARN: arn:aws:lambda:us-east-1::runtime:07a48df201798d627f2b950f03bb227aab4a655a1d019c3296406f95937e2525
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.112000 START RequestId: d455cfc4-7704-46df-901b-2a5cce9405be Version: $LATEST
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.114000 {
  "level": "INFO",
  "location": "hello:23",
    "message": "Hello world API - HTTP 200",
  "timestamp": "2023-02-03 14:59:51,113+0000",
  "service": "PowertoolsHelloWorld",
  "cold_start": true,
  "function_name": "sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "function_memory_size": "128",
  "function_arn": "arn:aws:lambda:us-east-1:111122223333:function:sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "function_request_id": "d455cfc4-7704-46df-901b-2a5cce9405be",
  "correlation_id": "e73f8aef-5e07-436e-a30b-63e4b23f0047",
  "xray_trace_id": "1-63dd2166-434a12c22e1307ff2114f299"
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.126000 {
  "_aws": {
    "Timestamp": 1675436391126,
    "CloudWatchMetrics": [
      {
        "Namespace": "Powertools",
        "Dimensions": [
          [
            "function_name",
            "service"
          ]
        ],
        "Metrics": [
          {
            "Name": "ColdStart",
            "Unit": "Count"
          }
        ]
      }
    ]
  },
  "function_name": "sam-app-HelloWorldFunction-YBg8yfYtOc9j",
  "service": "PowertoolsHelloWorld",
  "ColdStart": [
    1.0
  ]
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.126000 {
  "_aws": {
    "Timestamp": 1675436391126,
    "CloudWatchMetrics": [
      {
        "Namespace": "Powertools",
        "Dimensions": [
          [
            "service"
          ]
        ],
        "Metrics": [
          {
            "Name": "HelloWorldInvocations",
            "Unit": "Count"
          }
        ]
      }
    ]
  },
  "service": "PowertoolsHelloWorld",
  "HelloWorldInvocations": [
    1.0
  ]
}
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.128000 END RequestId: d455cfc4-7704-46df-901b-2a5cce9405be
2023/02/03/[$LATEST]ea9a64ec87294bf6bbc9026c05a01e04 2023-02-03T14:59:51.128000 REPORT RequestId: d455cfc4-7704-46df-901b-2a5cce9405be    Duration: 16.33 ms    Billed Duration: 17 ms    Memory Size: 128 MB    Max Memory Used: 64 MB    Init Duration: 739.46 ms    
XRAY TraceId: 1-63dd2166-434a12c22e1307ff2114f299    SegmentId: 3c5d18d735a1ced0    Sampled: true

This is a public API endpoint that is accessible over the internet. We recommend that you delete the endpoint after testing.

cdk destroy