import json
import unittest

from aws_lambda_powertools.event_handler import ALBResolver

app = ALBResolver()

@app.get("/poc", compress=True)
def test_poc():
    return {
        "statusCode": 200,
        "body": json.dumps({"foo": "bar"}),
    }

def lambda_handler(event, context):
    return app.resolve(event, context)

class TestLambdaHandler(unittest.TestCase):
    def setUp(self):
        self.event_base = {
            "httpMethod": "GET",
            "path": "/poc",
        }
        self.context = {}

    def test_lambda_handler_with_headers(self):
        event = self.event_base.copy()
        event["headers"] = {"accept-encoding": "gzip, deflate"}
        response = lambda_handler(event, self.context)
        assert "Content-Encoding" in response["headers"]

    def test_lambda_handler_with_multivalueheaders(self):
        event = self.event_base.copy()
        event["multiValueHeaders"] = {"accept-encoding": ["gzip", "deflate"]}
        response = lambda_handler(event, self.context)
        print(response)
        assert "multiValueHeaders" in response
        assert "headers" not in response
        assert "Content-Encoding" in response["multiValueHeaders"], response["multiValueHeaders"]

if __name__ == "__main__":
    unittest.main()

--
Result:

     assert "Content-Encoding" in response["multiValueHeaders"], response["multiValueHeaders"]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: defaultdict(<class 'list'>, {'Content-Type': ['application/json']})