Say I want to pass val1 and val2 in the URL string when making a GET request from my Api gateway endpoint to my Lambda function:

https://xyz.execute-api.amazonaws.com/prod/test?val1=5&val2=10

And I have a simple function that sums the two inputs, val1 and val2:

def lambda_handler(event, context):
    # How do I get at val1 and val2??
    return {'result': val1 + val2}

I’ve added val1 and val2 to URL Query String Parameters on the Method Request on the AWS API Gateway. But how do I access them inside the function?

After defining the query string parameters in Method Request section of the API Gateway, you then need to define a Mapping Template in the Method Execution section.

In the Method Execution section, select Mapping Templates and then click Add Mapping Template. Enter application/json for the Content Type and then create a mapping template that looks something like this:

{
    "va1": "$input.params('val1')",
    "val2": "$input.params('val2')"
}

This will tell API Gateway to take the input parameters (either passed on the path, or in headers, or in query parameters) called val1 and val2 and send them to the Lambda function in the event data as val1 and val2.

All the information can be retrieved from Event object.

For example :
The value of a variable foo can be retrieved from the event as : event["foo"].