Bug: REST response compression does not work when multiValueHeaders are enabled

[kimsappi]

Expected Behaviour

Response compression is possible when "multiValueHeaders" are enabled.

Current Behaviour

Compression does not work under any circumstances when "multiValueHeaders" are enabled.

Code snippet

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']})

Possible Solution

This seems to stem from the compressibility request header check only checking event.headers:

powertools-lambda-python/aws_lambda_powertools/event_handler/api_gateway.py

Line 919 in ed6d3d3

encoding = event.headers.get("accept-encoding", "")

event.resolved_headers_field is more generic, but requires some additional processing (e.g. "gzip, deflate" can be a single header value within the sequence of event.multi_value_headers).

Steps to Reproduce

Code snippet above

Powertools for AWS Lambda (Python) version

latest

AWS Lambda function runtime

3.11

Packaging format used

PyPi

Debugging logs

[kimsappi]

Something like this could work:

diff --git a/aws_lambda_powertools/event_handler/api_gateway.py b/aws_lambda_powertools/event_handler/api_gateway.py
index 0c4553f1..ff2bf01b 100644
--- a/aws_lambda_powertools/event_handler/api_gateway.py
+++ b/aws_lambda_powertools/event_handler/api_gateway.py
@@ -916,8 +916,16 @@ class ResponseBuilder(Generic[ResponseEventT]):
         bool
             True if compression is enabled and the "gzip" encoding is accepted, False otherwise.
         """
-        encoding = event.headers.get("accept-encoding", "")
-        if "gzip" in encoding:
+        encoding = event.resolved_headers_field.get("accept-encoding", "")
+
+        # Handle both string (single value headers) and list of strings (multi-value headers).
+        gzip_supported = False
+        if isinstance(encoding, list):
+            gzip_supported = any("gzip" in enc for enc in encoding)
+        else:
+            gzip_supported = "gzip" in encoding
+
+        if gzip_supported:
             if response_compression is not None:
                 return response_compression  # e.g., Response(compress=False/True))
             if route_compression:
diff --git a/tests/functional/event_handler/required_dependencies/test_api_gateway.py b/tests/functional/event_handler/required_dependencies/test_api_gateway.py
index f2fc514a..49e7a8f5 100644
--- a/tests/functional/event_handler/required_dependencies/test_api_gateway.py
+++ b/tests/functional/event_handler/required_dependencies/test_api_gateway.py
@@ -464,12 +464,19 @@ def test_override_route_compress_parameter():
     assert result["multiValueHeaders"].get("Content-Encoding") is None
 
 
-def test_response_with_compress_enabled():
+@pytest.mark.parametrize(
+    "headers", [
+        {"headers": {"Accept-Encoding": "deflate, gzip"}},
+        {"multiValueHeaders": {"Accept-Encoding": ["deflate, gzip"]}},
+        {"multiValueHeaders": {"Accept-Encoding": ["deflate", "gzip"]}},
+    ],
+)
+def test_response_with_compress_enabled(headers: dict):
     # GIVEN a function
     # AND an event with a "Accept-Encoding" that include gzip
     # AND the Response object with compress=True
     app = ApiGatewayResolver()
-    mock_event = {"path": "/my/request", "httpMethod": "GET", "headers": {"Accept-Encoding": "deflate, gzip"}}
+    mock_event = {"path": "/my/request", "httpMethod": "GET", **headers}
     expected_value = '{"test": "value"}'
 
     @app.get("/my/request")

But I'm having some issues running the tests and I definitely haven't checked any style guides or anything like that.

Splitting the header values by , would also make sense and checking for the exact string (stripped), since it's possible that there's going to be a gzip2 or something at some point. But then MDN suggests that this sort of thing is possible: Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5. So to do that, one would need to read the standard.

[dreamorosi]

@sthulb can you take a look at this one?

[leandrodamascena]

Hey @kimsappi I can confirm this bug! I think if we just change this line is enough:

encoding = event.resolved_headers_field.get("accept-encoding", "")

Do you want to submit a PR to fix this? I'd love your contribution here.

If not, I'll fix this by Monday.

[kimsappi]

I think if we just change this line is enough:
encoding = event.resolved_headers_field.get("accept-encoding", "")

It would be for the minimal example in the issue post (if accept-encoding: gzip is a separate header), but e.g. browsers commonly send accept-encoding: gzip, deflate, br, zstd, so we can't rely on "gzip" being a lone value in a list of multi-value headers.

Made a pull request. Feel free to edit it however you'd like. If the type safety was better (resolved_headers_field could return dict[str, str | list[str]]), that would make it a lot nicer.