fix(data_classes): return None from CloudWatchAlarmMetric.metric_stat for expression entries

aws_lambda_powertools/utilities/data_classes/cloud_watch_alarm_event.py

@@ -90,8 +90,15 @@ def return_data(self) -> bool:
         return self["returnData"]
 
     @property
-    def metric_stat(self) -> CloudWatchAlarmMetricStat:
-        return CloudWatchAlarmMetricStat(self["metricStat"])
+    def metric_stat(self) -> CloudWatchAlarmMetricStat | None:
+        """
+        Optional metric stat of the alarm metric.
+
+        Metric math alarms list expression entries alongside metric entries, and an
+        expression entry has no metricStat.
+        """
+        metric_stat = self.get("metricStat")
+        return CloudWatchAlarmMetricStat(metric_stat) if metric_stat else None
 
 
 class CloudWatchAlarmMetricStat(DictWrapper):

tests/unit/data_classes/required_dependencies/test_cloud_watch_alarm_event.py

@@ -57,6 +57,23 @@ def test_cloud_watch_alarm_event_single_metric():
     assert isinstance(metric_1.metric_stat.metric, dict)
 
 
+def test_cloud_watch_alarm_metric_stat_is_none_for_expression_entries():
+    raw_event = load_event("cloudWatchAlarmEventSingleMetric.json")
+    parsed_event = CloudWatchAlarmEvent(raw_event)
+    metrics = parsed_event.alarm_data.configuration.metrics
+
+    # GIVEN a metric math alarm, whose expression entries carry no metricStat key
+    expression_metric = metrics[0]
+    assert expression_metric.expression is not None
+    assert "metricStat" not in raw_event["alarmData"]["configuration"]["metrics"][0]
+
+    # THEN metric_stat must return None like expression and label do, not raise KeyError
+    assert expression_metric.metric_stat is None
+
+    # AND iterating every metric's metric_stat must be safe
+    assert [metric.metric_stat is not None for metric in metrics] == [False, True, True]
+
+
 def test_cloud_watch_alarm_event_composite_metric():
     raw_event = load_event("cloudWatchAlarmEventCompositeMetric.json")
     parsed_event = CloudWatchAlarmEvent(raw_event)