A DisableModules fixture that removes modules from path

This is a refactor of a fixture we use in keystonemiddleware for
removing modules from the python path.

Because of the way python works this typically means it only works on
modules that are imported inline or are reloaded manually.

Change-Id: Ib384e1910ca292fce81ccdd4ff857b64ee8d5a21

oslotest/modules.py

@@ -0,0 +1,50 @@
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import sys
+
+import fixtures
+
+
+class DisableModuleFixture(fixtures.Fixture):
+    """A fixture to provide support for unloading/disabling modules."""
+
+    def __init__(self, module, *args, **kwargs):
+        super(DisableModuleFixture, self).__init__(*args, **kwargs)
+        self.module = module
+
+    def setUp(self):
+        """Ensure ImportError for the specified module."""
+        super(DisableModuleFixture, self).setUp()
+
+        cleared_modules = {}
+
+        for name in list(sys.modules.keys()):
+            if name == self.module or name.startswith(self.module + '.'):
+                cleared_modules[name] = sys.modules.pop(name)
+
+        finder = _NoModuleFinder(self.module)
+        sys.meta_path.insert(0, finder)
+
+        self.addCleanup(sys.meta_path.remove, finder)
+        self.addCleanup(sys.modules.update, cleared_modules)
+
+
+class _NoModuleFinder(object):
+    """Disallow further imports of 'module'."""
+
+    def __init__(self, module):
+        self.module = module
+
+    def find_module(self, fullname, path):
+        if fullname == self.module or fullname.startswith(self.module + '.'):
+            raise ImportError

oslotest/tests/unit/test_modules.py

@@ -0,0 +1,28 @@
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+from oslotest import base
+from oslotest import modules
+
+
+class CreateDisableModuleTest(base.BaseTestCase):
+
+    def test_disable_module(self):
+
+        s = __import__('sys')
+        self.assertTrue(s)
+
+        with modules.DisableModuleFixture('sys'):
+            self.assertRaises(ImportError, __import__, 'sys')
+
+        s = __import__('sys')
+        self.assertTrue(s)