Implement override_config decorator for use in testing

Subclass django.test.utils.override_settings for similar pattern
Allow use as both TestCase decorator and context manager
Support Django versions 1.4 to 1.8

constance/test/__init__.py

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+from .utils import override_config

constance/test/utils.py

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+from functools import wraps
+
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+from .. import config
+
+__all__ = ('override_config',)
+
+
+class override_config(override_settings):
+    """Decorator to modify constance setting for TestCase.
+
+    Based on django.test.utils.override_settings.
+    """
+    def __init__(self, **kwargs):
+        super(override_config, self).__init__(**kwargs)
+        self.original_values = {}
+
+    def __call__(self, test_func):
+        """Modify the decorated function to override config values."""
+        if isinstance(test_func, type):
+            if not issubclass(test_func, SimpleTestCase):
+                raise Exception(
+                    "Only subclasses of Django SimpleTestCase can be "
+                    "decorated with override_config")
+            return self.modify_test_case(test_func)
+        else:
+            @wraps(test_func)
+            def inner(*args, **kwargs):
+                with self:
+                    return test_func(*args, **kwargs)
+        return inner
+
+    def modify_test_case(self, test_case):
+        """Override the config by modifying TestCase methods.
+
+        This method follows the Django <= 1.6 method of overriding the
+        _pre_setup and _post_teardown hooks rather than modifying the TestCase
+        itself.
+        """
+        original_pre_setup = test_case._pre_setup
+        original_post_teardown = test_case._post_teardown
+
+        def _pre_setup(inner_self):
+            self.enable()
+            original_pre_setup(inner_self)
+
+        def _post_teardown(inner_self):
+            original_post_teardown(inner_self)
+            self.disable()
+
+        test_case._pre_setup = _pre_setup
+        test_case._post_teardown = _post_teardown
+
+        return test_case
+
+    def enable(self):
+        """Store original config values and set overridden values."""
+        # Store the original values to an instance variable
+        for config_key in self.options.keys():
+            self.original_values[config_key] = getattr(config, config_key)
+
+        # Update config with the overriden values
+        self.unpack_values(self.options)
+
+    def disable(self):
+        """Set original values to the config."""
+        self.unpack_values(self.original_values)
+
+    @staticmethod
+    def unpack_values(options):
+        """Unpack values from the given dict to config."""
+        for name, value in options.items():
+            setattr(config, name, value)