Fix error when APIGateway headers are None (#259)

* Don't error when headers are None

* Fix formatting of file

Author: swfunc

datadog_lambda/tracing.py

@@ -174,7 +174,7 @@ def extract_context_from_http_event_or_context(event, lambda_context):
 
     Falls back to lambda context if no trace data is found in the `headers`
     """
-    headers = event.get("headers", {})
+    headers = event.get("headers", {}) or {}
     lowercase_headers = {k.lower(): v for k, v in headers.items()}
 
     trace_id = lowercase_headers.get(TraceHeader.TRACE_ID)

tests/test_tracing.py

@@ -1237,3 +1237,11 @@ def test_mark_trace_as_error_for_5xx_responses_sends_error_metric_and_set_error_
         )
         mock_submit_errors_metric.assert_called_once()
         self.assertEqual(1, mock_span.error)
+
+    def test_no_error_with_nonetype_headers(self):
+        lambda_ctx = get_mock_context()
+        ctx, source = extract_dd_trace_context(
+            {"headers": None},
+            lambda_ctx,
+        )
+        self.assertEqual(ctx, None)