From b6789a2cc7d7ab9c2bd3fdd5b95d00e6fa20758d Mon Sep 17 00:00:00 2001
From: Tomoya Iwata <iwata.tomoya@classmethod.jp>
Date: Tue, 21 Apr 2020 14:11:53 +0900
Subject: [PATCH 1/2] Added existence check of target thing to IoT
 ListThingPrincipals fix #2910

---
 moto/iot/exceptions.py     | 4 ++--
 moto/iot/models.py         | 8 ++++++++
 tests/test_iot/test_iot.py | 7 +++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/moto/iot/exceptions.py b/moto/iot/exceptions.py
index d114a12ad..7a578c221 100644
--- a/moto/iot/exceptions.py
+++ b/moto/iot/exceptions.py
@@ -7,10 +7,10 @@ class IoTClientError(JsonRESTError):
 
 
 class ResourceNotFoundException(IoTClientError):
-    def __init__(self):
+    def __init__(self, msg=None):
         self.code = 404
         super(ResourceNotFoundException, self).__init__(
-            "ResourceNotFoundException", "The specified resource does not exist"
+            "ResourceNotFoundException", msg or "The specified resource does not exist"
         )
 
 
diff --git a/moto/iot/models.py b/moto/iot/models.py
index de4383b96..51a23b6c6 100644
--- a/moto/iot/models.py
+++ b/moto/iot/models.py
@@ -805,6 +805,14 @@ class IoTBackend(BaseBackend):
         return thing_names
 
     def list_thing_principals(self, thing_name):
+
+        things = [_ for _ in self.things.values() if _.thing_name == thing_name]
+        if len(things) == 0:
+            raise ResourceNotFoundException(
+                "Failed to list principals for thing %s because the thing does not exist in your account"
+                % thing_name
+            )
+
         principals = [
             k[0] for k, v in self.principal_things.items() if k[1] == thing_name
         ]
diff --git a/tests/test_iot/test_iot.py b/tests/test_iot/test_iot.py
index f8c4f579c..f3c151714 100644
--- a/tests/test_iot/test_iot.py
+++ b/tests/test_iot/test_iot.py
@@ -728,6 +728,13 @@ def test_principal_thing():
     res = client.list_thing_principals(thingName=thing_name)
     res.should.have.key("principals").which.should.have.length_of(0)
 
+    with assert_raises(ClientError) as e:
+        client.list_thing_principals(thingName='xxx')
+
+    e.exception.response["Error"]["Code"].should.equal("ResourceNotFoundException")
+    e.exception.response["Error"]["Message"].should.equal(
+        "Failed to list principals for thing xxx because the thing does not exist in your account"
+    )
 
 @mock_iot
 def test_delete_principal_thing():

From d9b782be0a6944426347378345b2289732a2c7d9 Mon Sep 17 00:00:00 2001
From: Tomoya Iwata <iwata.tomoya@classmethod.jp>
Date: Tue, 21 Apr 2020 14:43:04 +0900
Subject: [PATCH 2/2] fix lint

---
 tests/test_iot/test_iot.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/test_iot/test_iot.py b/tests/test_iot/test_iot.py
index f3c151714..2f43de5b9 100644
--- a/tests/test_iot/test_iot.py
+++ b/tests/test_iot/test_iot.py
@@ -729,13 +729,14 @@ def test_principal_thing():
     res.should.have.key("principals").which.should.have.length_of(0)
 
     with assert_raises(ClientError) as e:
-        client.list_thing_principals(thingName='xxx')
+        client.list_thing_principals(thingName="xxx")
 
     e.exception.response["Error"]["Code"].should.equal("ResourceNotFoundException")
     e.exception.response["Error"]["Message"].should.equal(
         "Failed to list principals for thing xxx because the thing does not exist in your account"
     )
 
+
 @mock_iot
 def test_delete_principal_thing():
     client = boto3.client("iot", region_name="ap-northeast-1")