Add TestExtensionManager

Create a special extension manager class that uses pre-configured
extension instances instead of loading them from a plugin set.
This makes setting up controlled environments for testing much
easier.

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

docs/source/history.rst

@@ -2,6 +2,11 @@
  History
 =========
 
+0.5
+
+  - Add :class:`~stevedore.tests.manager.TestExtensionManager` for
+    writing tests for classes that use extension managers.
+
 0.4
 
   - Removed the ``name`` argument to plugin constructors.

docs/source/managers.rst

@@ -63,3 +63,9 @@ Extension
    :members:
    :show-inheritance:
 
+TestExtensionManager
+====================
+
+.. autoclass:: stevedore.tests.manager.TestExtensionManager
+   :members:
+   :show-inheritance:

stevedore/extension.py

@@ -52,7 +52,12 @@ class ExtensionManager(object):
                  invoke_args=(),
                  invoke_kwds={}):
         self.namespace = namespace
-        self.extensions = []
+        self.extensions = self._load_plugins(invoke_on_load,
+                                             invoke_args,
+                                             invoke_kwds)
+
+    def _load_plugins(self, invoke_on_load, invoke_args, invoke_kwds):
+        extensions = []
         for ep in pkg_resources.iter_entry_points(self.namespace):
             LOG.debug('found extension %r', ep)
             try:
@@ -62,13 +67,13 @@ class ExtensionManager(object):
                                             invoke_kwds,
                                             )
                 if ext:
-                    self.extensions.append(ext)
+                    extensions.append(ext)
             except KeyboardInterrupt:
                 raise
             except Exception as err:
                 LOG.error('Could not load %r: %s', ep.name, err)
                 LOG.exception(err)
-        return
+        return extensions
 
     def _load_one_plugin(self, ep, invoke_on_load, invoke_args, invoke_kwds):
         plugin = ep.load()

stevedore/tests/manager.py

@@ -0,0 +1,50 @@
+"""TestExtensionManager
+
+Extension manager used only for testing.
+"""
+
+import logging
+
+from stevedore import extension
+
+
+LOG = logging.getLogger(__name__)
+
+
+class TestExtensionManager(extension.ExtensionManager):
+    """ExtensionManager that is explicitly initialized for tests.
+
+    :param extensions: Pre-configured Extension instances to use
+                       instead of loading them from entry points.
+    :type extensions: list of :class:`~stevedore.extension.Extension`
+    :param namespace: The namespace for the entry points.
+    :type namespace: str
+    :param invoke_on_load: Boolean controlling whether to invoke the
+        object returned by the entry point after the driver is loaded.
+    :type invoke_on_load: bool
+    :param invoke_args: Positional arguments to pass when invoking
+        the object returned by the entry point. Only used if invoke_on_load
+        is True.
+    :type invoke_args: tuple
+    :param invoke_kwds: Named arguments to pass when invoking
+        the object returned by the entry point. Only used if invoke_on_load
+        is True.
+    :type invoke_kwds: dict
+    """
+
+    def __init__(self, extensions,
+                 namespace='test',
+                 invoke_on_load=False,
+                 invoke_args=(),
+                 invoke_kwds={}):
+        super(TestExtensionManager, self).__init__(namespace,
+                                                   invoke_on_load,
+                                                   invoke_args,
+                                                   invoke_kwds,
+                                                   )
+        self.extensions = extensions
+
+    def _load_plugins(self, invoke_on_load,
+                      invoke_args,
+                      invoke_kwds):
+        return []