Error listing Lambda layers after layer deletion (#5376)

moto/awslambda/models.py

@@ -364,12 +364,16 @@ class Layer(object):
         self.layer_versions.pop(str(layer_version), None)
 
     def to_dict(self):
+        if not self.layer_versions:
+            return {}
+
+        last_key = sorted(self.layer_versions.keys(), key=lambda version: int(version))[
+            -1
+        ]
         return {
             "LayerName": self.name,
             "LayerArn": self.layer_arn,
-            "LatestMatchingVersion": self.layer_versions[
-                str(self._latest_version)
-            ].get_layer_version(),
+            "LatestMatchingVersion": self.layer_versions[last_key].get_layer_version(),
         }
 
 
@@ -1242,7 +1246,9 @@ class LayerStorage(object):
         self._layers[layer_version.name].attach_version(layer_version)
 
     def list_layers(self):
-        return [layer.to_dict() for layer in self._layers.values()]
+        return [
+            layer.to_dict() for layer in self._layers.values() if layer.layer_versions
+        ]
 
     def delete_layer_version(self, layer_name, layer_version):
         self._layers[layer_name].delete_version(layer_version)

tests/test_awslambda/test_lambda_layers.py

@@ -230,3 +230,69 @@ def test_delete_layer_version():
 
     result = conn.list_layer_versions(LayerName=layer_name)["LayerVersions"]
     result.should.equal([])
+
+
+@mock_lambda
+@mock_s3
+def test_get_layer_with_no_layer_versions():
+    def get_layer_by_layer_name_from_list_of_layer_dicts(layer_name, layer_list):
+        for layer in layer_list:
+            if layer["LayerName"] == layer_name:
+                return layer
+        return None
+
+    conn = boto3.client("lambda", _lambda_region)
+    layer_name = str(uuid4())[0:6]
+
+    # Publish a new Layer and assert Layer exists and only version 1 is there
+    conn.publish_layer_version(
+        LayerName=layer_name,
+        Content={"ZipFile": get_test_zip_file1()},
+    )
+    assert (
+        get_layer_by_layer_name_from_list_of_layer_dicts(
+            layer_name, conn.list_layers()["Layers"]
+        )["LatestMatchingVersion"]["Version"]
+        == 1
+    )
+
+    # Add a new version of that Layer then delete that version
+    conn.publish_layer_version(
+        LayerName=layer_name,
+        Content={"ZipFile": get_test_zip_file1()},
+    )
+    assert (
+        get_layer_by_layer_name_from_list_of_layer_dicts(
+            layer_name, conn.list_layers()["Layers"]
+        )["LatestMatchingVersion"]["Version"]
+        == 2
+    )
+
+    conn.delete_layer_version(LayerName=layer_name, VersionNumber=2)
+    assert (
+        get_layer_by_layer_name_from_list_of_layer_dicts(
+            layer_name, conn.list_layers()["Layers"]
+        )["LatestMatchingVersion"]["Version"]
+        == 1
+    )
+
+    # Delete the last layer_version and check that the Layer is still in the LayerStorage
+    conn.delete_layer_version(LayerName=layer_name, VersionNumber=1)
+    assert (
+        get_layer_by_layer_name_from_list_of_layer_dicts(
+            layer_name, conn.list_layers()["Layers"]
+        )
+        is None
+    )
+
+    # Assert _latest_version didn't decrement
+    conn.publish_layer_version(
+        LayerName=layer_name,
+        Content={"ZipFile": get_test_zip_file1()},
+    )
+    assert (
+        get_layer_by_layer_name_from_list_of_layer_dicts(
+            layer_name, conn.list_layers()["Layers"]
+        )["LatestMatchingVersion"]["Version"]
+        == 3
+    )