diff --git a/scrapy/settings/__init__.py b/scrapy/settings/__init__.py
index b0f59ccc7..e62bdd08e 100644
--- a/scrapy/settings/__init__.py
+++ b/scrapy/settings/__init__.py
@@ -49,14 +49,11 @@ class SettingsAttribute(object):
 
     def set(self, value, priority):
         """Sets value if priority is higher or equal than current priority."""
-        if isinstance(self.value, BaseSettings):
-            # Ignore self.priority if self.value has per-key priorities
-            self.value.update(value, priority)
-            self.priority = max(self.value.maxpriority(), priority)
-        else:
-            if priority >= self.priority:
-                self.value = value
-                self.priority = priority
+        if priority >= self.priority:
+            if isinstance(self.value, BaseSettings):
+                value = BaseSettings(value, priority=priority)
+            self.value = value
+            self.priority = priority
 
     def __str__(self):
         return "<SettingsAttribute value={self.value!r} " \
diff --git a/tests/test_cmdline/__init__.py b/tests/test_cmdline/__init__.py
index 5192fb0fa..c2de4fbc8 100644
--- a/tests/test_cmdline/__init__.py
+++ b/tests/test_cmdline/__init__.py
@@ -1,10 +1,11 @@
-import os
 import json
-import sys
-import shutil
+import os
 import pstats
-import tempfile
+import shutil
+import six
 from subprocess import Popen, PIPE
+import sys
+import tempfile
 import unittest
 try:
     from cStringIO import StringIO
@@ -57,14 +58,14 @@ class CmdlineTest(unittest.TestCase):
             shutil.rmtree(path)
 
     def test_override_dict_settings(self):
+        EXT_PATH = "tests.test_cmdline.extensions.DummyExtension"
+        EXTENSIONS = {EXT_PATH: 200}
         settingsstr = self._execute('settings', '--get', 'EXTENSIONS', '-s',
-                                    ('EXTENSIONS={"tests.test_cmdline.extensions.TestExtension": '
-                                     '100, "tests.test_cmdline.extensions.DummyExtension": 200}'))
+                                    'EXTENSIONS=' + json.dumps(EXTENSIONS))
         # XXX: There's gotta be a smarter way to do this...
         self.assertNotIn("...", settingsstr)
         for char in ("'", "<", ">", 'u"'):
             settingsstr = settingsstr.replace(char, '"')
         settingsdict = json.loads(settingsstr)
-        self.assertIn('tests.test_cmdline.extensions.DummyExtension', settingsdict)
-        self.assertIn('value=200', settingsdict['tests.test_cmdline.extensions.DummyExtension'])
-        self.assertIn('value=100', settingsdict['tests.test_cmdline.extensions.TestExtension'])
+        six.assertCountEqual(self, settingsdict.keys(), EXTENSIONS.keys())
+        self.assertIn('value=200', settingsdict[EXT_PATH])
diff --git a/tests/test_settings/__init__.py b/tests/test_settings/__init__.py
index 8d98d2cec..44b9b6df3 100644
--- a/tests/test_settings/__init__.py
+++ b/tests/test_settings/__init__.py
@@ -37,21 +37,22 @@ class SettingsAttributeTest(unittest.TestCase):
         self.assertEqual(self.attribute.value, 'value')
         self.assertEqual(self.attribute.priority, 10)
 
-    def test_set_per_key_priorities(self):
-        attribute = SettingsAttribute(
-            BaseSettings({'one': 10, 'two': 20}, 0), 0)
+    def test_overwrite_basesettings(self):
+        original_dict = {'one': 10, 'two': 20}
+        original_settings = BaseSettings(original_dict, 0)
+        attribute = SettingsAttribute(original_settings, 0)
 
-        new_dict = {'one': 11, 'two': 21}
+        new_dict = {'three': 11, 'four': 21}
         attribute.set(new_dict, 10)
-        self.assertEqual(attribute.value['one'], 11)
-        self.assertEqual(attribute.value['two'], 21)
+        self.assertIsInstance(attribute.value, BaseSettings)
+        six.assertCountEqual(self, attribute.value, new_dict)
+        six.assertCountEqual(self, original_settings, original_dict)
 
-        new_settings = BaseSettings()
-        new_settings.set('one', 12, 20)
-        new_settings.set('two', 12, 0)
-        attribute.set(new_settings, 0)
-        self.assertEqual(attribute.value['one'], 12)
-        self.assertEqual(attribute.value['two'], 21)
+        new_settings = BaseSettings({'five': 12}, 0)
+        attribute.set(new_settings, 0)  # Insufficient priority
+        six.assertCountEqual(self, attribute.value, new_dict)
+        attribute.set(new_settings, 10)
+        six.assertCountEqual(self, attribute.value, new_settings)
 
     def test_repr(self):
         self.assertEqual(repr(self.attribute),