awslambda: create_function() should fail on duplicate (#6626)

moto/awslambda/exceptions.py

@@ -14,6 +14,14 @@ class CrossAccountNotAllowed(LambdaClientError):
         )
 
 
+class FunctionAlreadyExists(LambdaClientError):
+    code = 409
+
+    def __init__(self, function_name: str) -> None:
+        message = f"Function already exist: {function_name}"
+        super().__init__("ResourceConflictException", message)
+
+
 class InvalidParameterValueException(LambdaClientError):
     def __init__(self, message: str):
         super().__init__("InvalidParameterValueException", message)

moto/awslambda/responses.py

@@ -6,6 +6,7 @@ from urllib.parse import unquote
 from moto.core.utils import path_url
 from moto.utilities.aws_headers import amz_crc32, amzn_request_id
 from moto.core.responses import BaseResponse, TYPE_RESPONSE
+from .exceptions import FunctionAlreadyExists, UnknownFunctionException
 from .models import lambda_backends, LambdaBackend
 
 
@@ -316,9 +317,14 @@ class LambdaResponse(BaseResponse):
         return 200, {}, json.dumps(result)
 
     def _create_function(self) -> TYPE_RESPONSE:
+        function_name = self.json_body["FunctionName"].rsplit(":", 1)[-1]
+        try:
+            self.backend.get_function(function_name, None)
+        except UnknownFunctionException:
             fn = self.backend.create_function(self.json_body)
             config = fn.get_configuration(on_create=True)
             return 201, {}, json.dumps(config)
+        raise FunctionAlreadyExists(function_name)
 
     def _create_function_url_config(self) -> TYPE_RESPONSE:
         function_name = unquote(self.path.split("/")[-2])

tests/test_awslambda/test_lambda.py

@@ -1236,7 +1236,8 @@ def test_create_function_with_already_exists():
         Publish=True,
     )
 
-    response = conn.create_function(
+    with pytest.raises(ClientError) as exc:
+        conn.create_function(
             FunctionName=function_name,
             Runtime="python2.7",
             Role=get_role_name(),
@@ -1248,7 +1249,7 @@ def test_create_function_with_already_exists():
             Publish=True,
         )
 
-    assert response["FunctionName"] == function_name
+    assert exc.value.response["Error"]["Code"] == "ResourceConflictException"
 
 
 @mock_lambda

tests/test_awslambda/test_lambda_layers.py

@@ -115,6 +115,7 @@ def test_get_lambda_layers():
     assert result["LayerVersions"] == []
 
     # Test create function with non existant layer version
+    function_name = str(uuid4())[0:6]  # Must be different than above
     with pytest.raises(ClientError) as exc:
         conn.create_function(
             FunctionName=function_name,