Minor refactoring to avoid code duplication

moto/ec2/models.py

@@ -141,8 +141,7 @@ class InstanceBackend(object):
 
     def start_instances(self, instance_ids):
         started_instances = []
-        for instance in self.all_instances():
-            if instance.id in instance_ids:
+        for instance in self.get_multi_instances_by_id(instance_ids):
             instance.start()
             started_instances.append(instance)
 
@@ -150,8 +149,7 @@ class InstanceBackend(object):
 
     def stop_instances(self, instance_ids):
         stopped_instances = []
-        for instance in self.all_instances():
-            if instance.id in instance_ids:
+        for instance in self.get_multi_instances_by_id(instance_ids):
             instance.stop()
             stopped_instances.append(instance)
 
@@ -159,8 +157,7 @@ class InstanceBackend(object):
 
     def terminate_instances(self, instance_ids):
         terminated_instances = []
-        for instance in self.all_instances():
-            if instance.id in instance_ids:
+        for instance in self.get_multi_instances_by_id(instance_ids):
             instance.terminate()
             terminated_instances.append(instance)
 
@@ -168,8 +165,7 @@ class InstanceBackend(object):
 
     def reboot_instances(self, instance_ids):
         rebooted_instances = []
-        for instance in self.all_instances():
-            if instance.id in instance_ids:
+        for instance in self.get_multi_instances_by_id(instance_ids):
             instance.reboot()
             rebooted_instances.append(instance)
 
@@ -192,6 +188,20 @@ class InstanceBackend(object):
                 instances.append(instance)
         return instances
 
+    def get_multi_instances_by_id(self, instance_ids):
+        """
+        :param instance_ids: A string list with instance ids
+        :return: A list with instance objects
+        """
+        result = []
+
+        for reservation in self.all_reservations():
+            for instance in reservation.instances:
+                if instance.id in instance_ids:
+                    result.append(instance)
+
+        return result
+
     def get_instance_by_id(self, instance_id):
         for reservation in self.all_reservations():
             for instance in reservation.instances: