handle missing subnet tags

src/mu/libs/ec2.py

@@ -10,7 +10,7 @@ def describe_subnets(
     resp = ec2.describe_subnets()
     retval = {}
     for subnet in resp['Subnets']:
-        names = [pair['Value'] for pair in subnet['Tags'] if pair['Key'] == name_tag_key]
+        names = [pair['Value'] for pair in subnet.get('Tags', []) if pair['Key'] == name_tag_key]
         name = names[0] if names else ''
         retval_key = name if name else subnet['SubnetId']
         if not filter_prefix or name.startswith(filter_prefix):

src/mu_tests/test_ec2.py

@@ -0,0 +1,28 @@
+from unittest import mock
+
+from mu.libs import ec2
+
+
+@mock.patch('boto3.client')
+def test_describe_subnets_handles_untagged_subnets(mock_client):
+    mock_client.return_value.describe_subnets.return_value = {
+        'Subnets': [
+            {
+                'SubnetId': 'subnet-untagged',
+            },
+            {
+                'SubnetId': 'subnet-tagged',
+                'Tags': [{'Key': 'Name', 'Value': 'nest-dev-private-a'}],
+            },
+        ],
+    }
+
+    result = ec2.describe_subnets(None, name_tag_key='Name')
+
+    assert result == {
+        'subnet-untagged': {'SubnetId': 'subnet-untagged'},
+        'nest-dev-private-a': {
+            'SubnetId': 'subnet-tagged',
+            'Tags': [{'Key': 'Name', 'Value': 'nest-dev-private-a'}],
+        },
+    }