Add Settings.copy, freeze and frozencopy method

docs/topics/api.rst

@@ -264,6 +264,30 @@ Settings API
        :param default: the value to return if no setting is found
        :type default: any
 
+    .. method:: copy()
+
+       Make a deep copy of current settings.
+
+       This method returns a new instance of the :class:`Settings` class,
+       populated with the same values and their priorities.
+
+       Modifications to the new object won't be reflected on the original
+       settings.
+
+    .. method:: freeze()
+
+       Disable further changes to the current settings.
+
+       After calling this method, the present state of the settings will become
+       immutable. Trying to change values through the :meth:`~set` method and
+       its variants won't be possible and will be alerted.
+
+    .. method:: frozencopy()
+
+       Return an immutable copy of the current settings.
+
+       Alias for a :meth:`~freeze` call in the object returned by :meth:`copy`
+
 .. _topics-api-signals:
 
 Signals API

scrapy/settings/__init__.py

@@ -1,5 +1,6 @@
 import six
 import json
+import copy
 import warnings
 from collections import MutableMapping
 from importlib import import_module
@@ -46,6 +47,7 @@ class SettingsAttribute(object):
 class Settings(object):
 
     def __init__(self, values=None, priority='project'):
+        self.frozen = False
         self.attributes = {}
         self.setmodule(default_settings, priority='default')
         if values is not None:
@@ -93,6 +95,7 @@ class Settings(object):
         raise ValueError("Cannot convert value for setting '%s' to dict: '%s'" % (name, value))
 
     def set(self, name, value, priority='project'):
+        assert not self.frozen, "Trying to modify an immutable Settings object"
         if isinstance(priority, six.string_types):
             priority = SETTINGS_PRIORITIES[priority]
         if name not in self.attributes:
@@ -101,16 +104,29 @@ class Settings(object):
             self.attributes[name].set(value, priority)
 
     def setdict(self, values, priority='project'):
+        assert not self.frozen, "Trying to modify an immutable Settings object"
         for name, value in six.iteritems(values):
             self.set(name, value, priority)
 
     def setmodule(self, module, priority='project'):
+        assert not self.frozen, "Trying to modify an immutable Settings object"
         if isinstance(module, six.string_types):
             module = import_module(module)
         for key in dir(module):
             if key.isupper():
                 self.set(key, getattr(module, key), priority)
 
+    def copy(self):
+        return copy.deepcopy(self)
+
+    def freeze(self):
+        self.frozen = True
+
+    def frozencopy(self):
+        copy = self.copy()
+        copy.freeze()
+        return copy
+
     @property
     def overrides(self):
         warnings.warn("`Settings.overrides` attribute is deprecated and won't "

tests/test_settings/__init__.py

@@ -190,6 +190,42 @@ class SettingsTest(unittest.TestCase):
         self.assertEqual(settings.getdict('TEST_DICT3', {'key1': 5}), {'key1': 5})
         self.assertRaises(ValueError, settings.getdict, 'TEST_LIST1')
 
+    def test_copy(self):
+        values = {
+            'TEST_BOOL': True,
+            'TEST_LIST': ['one', 'two'],
+            'TEST_LIST_OF_LISTS': [['first_one', 'first_two'],
+                                   ['second_one', 'second_two']]
+        }
+        self.settings.setdict(values)
+        copy = self.settings.copy()
+        self.settings.set('TEST_BOOL', False)
+        self.assertTrue(copy.get('TEST_BOOL'))
+
+        test_list = self.settings.get('TEST_LIST')
+        test_list.append('three')
+        self.assertListEqual(copy.get('TEST_LIST'), ['one', 'two'])
+
+        test_list_of_lists = self.settings.get('TEST_LIST_OF_LISTS')
+        test_list_of_lists[0].append('first_three')
+        self.assertListEqual(copy.get('TEST_LIST_OF_LISTS')[0],
+                             ['first_one', 'first_two'])
+
+    def test_freeze(self):
+        self.settings.freeze()
+        with self.assertRaises(AssertionError) as cm:
+            self.settings.set('TEST_BOOL', False)
+            self.assertEqual(str(cm.exception),
+                             "Trying to modify an immutable Settings object")
+
+    def test_frozencopy(self):
+        with mock.patch.object(self.settings, 'copy') as mock_copy:
+            with mock.patch.object(mock_copy, 'freeze') as mock_freeze:
+                mock_object = self.settings.frozencopy()
+                mock_copy.assert_call_once()
+                mock_freeze.assert_call_once()
+                self.assertEqual(mock_object, mock_copy.return_value)
+
     def test_deprecated_attribute_overrides(self):
         self.settings.set('BAR', 'fuz', priority='cmdline')
         with warnings.catch_warnings(record=True) as w: