EC2: create_fleet() now correctly handles Overrides with a single val…

…ue (#8316)

moto/ec2/responses/fleets.py

@@ -33,6 +33,22 @@ def create_fleet(self) -> str:
         launch_template_configs = self._get_multi_param(
             param_prefix="LaunchTemplateConfigs"
         )
+        # Our multi_param function doesn't understand when to return a list, a string, or a dictionary
+        # So we'll have to help it a little, by explicitly asking for a dict if we're getting anything else back
+        for idx, config in enumerate(launch_template_configs, start=1):
+            if (overrides := config.get("Overrides")) and not isinstance(
+                overrides, dict
+            ):
+                config["Overrides"] = self._get_multi_param(
+                    f"LaunchTemplateConfigs.{idx}.Overrides",
+                    skip_result_conversion=True,
+                )
+            if (spec := config.get("LaunchTemplateSpecification")) and not isinstance(
+                spec, dict
+            ):
+                config["LaunchTemplateSpecification"] = self._get_multi_param_dict(
+                    f"LaunchTemplateConfigs.{idx}.LaunchTemplateSpecification"
+                )
 
         excess_capacity_termination_policy = self._get_param(
             "ExcessCapacityTerminationPolicy"

tests/test_ec2/test_fleets.py

@@ -328,6 +328,35 @@ def test_create_fleet_request_with_tags():
             assert tag in instance["Tags"]
 
 
+@mock_aws
+@pytest.mark.parametrize(
+    "overrides", [{"InstanceType": "t2.nano"}, {"AvailabilityZone": "us-west-1"}]
+)
+def test_create_fleet_using_launch_template_config__overrides_single(overrides):
+    conn = boto3.client("ec2", region_name="us-east-2")
+
+    template_id, _ = get_launch_template(conn, instance_type="t2.medium")
+
+    fleet_id = conn.create_fleet(
+        LaunchTemplateConfigs=[
+            {
+                "LaunchTemplateSpecification": {"LaunchTemplateId": template_id},
+                # Verify we parse the incoming parameters correctly,
+                # even when only supplying a dictionary with a single key
+                "Overrides": [overrides],
+            },
+        ],
+        TargetCapacitySpecification={
+            "DefaultTargetCapacityType": "spot",
+            "TotalTargetCapacity": 1,
+        },
+    )["FleetId"]
+
+    fleet = conn.describe_fleets(FleetIds=[fleet_id])["Fleets"][0]
+    lt_config = fleet["LaunchTemplateConfigs"][0]
+    assert lt_config["Overrides"] == [overrides]
+
+
 @mock_aws
 def test_create_fleet_using_launch_template_config__overrides():
     conn = boto3.client("ec2", region_name="us-east-2")