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Define Lambda function handler in Python

The Lambda function handler is the method in your function code that processes events. When your function is invoked, Lambda runs the handler method. Your function runs until the handler returns a response, exits, or times out.

You can use the following general syntax when creating a function handler in Python:

def handler_name(event, context): 
    ...
    return some_value

Naming

The Lambda function handler name specified at the time that you create a Lambda function is derived from:

A function handler can be any name; however, the default name in the Lambda console is lambda_function.lambda_handler. This function handler name reflects the function name (lambda_handler) and the file where the handler code is stored (lambda_function.py).

If you create a function in the console using a different file name or function handler name, you must edit the default handler name.

How it works

When Lambda invokes your function handler, the Lambda runtime passes two arguments to the function handler:

Returning a value

Optionally, a handler can return a value. What happens to the returned value depends on the invocation type and the service that invoked the function. For example:

Examples

The following section shows examples of Python functions you can use with Lambda. If you use the Lambda console to author your function, you do not need to attach a .zip archive file to run the functions in this section. These functions use standard Python libraries which are included with the Lambda runtime you selected. For more information, see Lambda deployment packages.

Returning a message

The following example shows a function called lambda_handler. The function accepts user input of a first and last name, and returns a message that contains data from the event it received as input.

def lambda_handler(event, context):
    message = 'Hello {} {}!'.format(event['first_name'], event['last_name'])  
    return { 
        'message' : message
    }

You can use the following event data to invoke the function:

{
    "first_name": "John",
    "last_name": "Smith"
}

The response shows the event data passed as input:

{
    "message": "Hello John Smith!"
}

Parsing a response

The following example shows a function called lambda_handler. The function uses event data passed by Lambda at runtime. It parses the environment variable in AWS_REGION returned in the JSON response.

import os
import json
        
def lambda_handler(event, context):
    json_region = os.environ['AWS_REGION']
    return {
        "statusCode": 200,
        "headers": {
            "Content-Type": "application/json"
        },
        "body": json.dumps({
            "Region ": json_region
        })
    }

You can use any event data to invoke the function:

{
  "key1": "value1",
  "key2": "value2",
  "key3": "value3"
}

Lambda runtimes set several environment variables during initialization. For more information on the environment variables returned in the response at runtime, see Use Lambda environment variables to configure values in code.

The function in this example depends on a successful response (in 200) from the Invoke API. For more information on the Invoke API status, see the Invoke Response Syntax.

Returning a calculation

The following example shows a function called lambda_handler. The function accepts user input and returns a calculation to the user. For more information about this example, see the aws-doc-sdk-examples GitHub repository.

import logging

logger = logging.getLogger()
logger.setLevel(logging.INFO)

def lambda_handler(event, context):
    ...
    result = None
    action = event.get('action')
    if action == 'increment':
        result = event.get('number', 0) + 1
        logger.info('Calculated result of %s', result)
    else:
        logger.error("%s is not a valid action.", action)

    response = {'result': result}
    return response

You can use the following event data to invoke the function:

{
    "action": "increment",
    "number": 3
}