If you don't want to update your lambda function, just simulate APIGateway event object by boto3 client:

If your api looks like https://linktolambda/{id} (ex: https://linktolambda/123456)

You will invoke with this code:

        payload = { "pathParameters": { "id":"123456" } } 
        result = client.invoke(FunctionName=conf.lambda_function_name,
                    InvocationType='RequestResponse',                                      
                    Payload=json.dumps(payload))

Or your API look like https://linktolambda?id=123456

        payload = { "queryStringParameters": { "id":"123456" } } 
        result = client.invoke(FunctionName=conf.lambda_function_name,
                    InvocationType='RequestResponse',                                      
                    Payload=json.dumps(payload))

lets say your invoke_lambda is in my_lambda.py file. You want to use the patch annotation to mock the lambda_client, not the response. You can then set the return_value of the mocked_lambda_client to mocked_response.

# my_lambda.py
def get_attachment(my_id):
    payload = {"myId": my_id}
    response = lambda_client.invoke(
        FunctionName=os.environ["MY_LAMBDA"],
        Payload=json.dumps(payload),
    )
    return response.status_code

# test_my_lambda.py
@mock.patch("my_lambda.lambda_client")
def test_duck_response_200(mock_lambda_client):
    mocked_response = mock.Mock()
    mocked_response.status_code = 200

    mock_lambda_client.invoke.return_value = mocked_response
    response = get_attachment('some_id')
    assert response == 200

It appears that you are saying that Lambda function A is timing out when invoking Lambda function B.

A common scenario is that the timeout is caused by Lambda function A not having access to the Internet. The AWS API endpoint is on the Internet and Lambda function A requires Internet access to invoke Lambda function B.

To have Internet access, the Lambda function should either:

• NOT be connected to a VPC (which then automatically provides Internet access), or
• Be connected to a private subnet, and the VPC should have an Internet Gateway in a public subnet to grant access to the Internet, or
• Configure interface VPC endpoints for Lambda, which creates a direct connection between a VPC and the AWS Lambda API endpoint

Thank you everyone for the input. @Michael-sqlbot's comment about the AWS client library defaulting to sending requests to the local region is what helped me find the solution. For Python, the library is boto3. Having read the docs it was not clear how to set the region. It was this blog post that provided the (simple) answer:

client = boto3.client('lambda', region_name='us-west-2')

You are right Michael that the use case for one lambda to another between regions is convoluted. I'll leave this answer here in case any others who are new to boto3 encounter the same error when trying to get other resources (lambda to ec2, lambda to s3, etc) to work across regions.