add DriverManager

stevedore/__init__.py

@@ -1,3 +1,5 @@
 from .extension import ExtensionManager
 from .enabled import EnabledExtensionManager
 from .named import NamedExtensionManager
+from .hook import HookManager
+from .driver import DriverManager

stevedore/driver.py

@@ -0,0 +1,24 @@
+from .hook import HookManager
+
+
+class DriverManager(HookManager):
+    """Load a single plugin with a given name from the namespace.
+    """
+
+    def __init__(self, namespace, name,
+                 invoke_on_load=False, invoke_args=(), invoke_kwds={}):
+        super(HookManager, self).__init__(
+            namespace,
+            [name],
+            invoke_on_load=invoke_on_load,
+            invoke_args=invoke_args,
+            invoke_kwds=invoke_kwds,
+            )
+        if not self.extensions:
+            raise RuntimeError('No %r driver found' % namespace)
+        if len(self.extensions) > 1:
+            raise RuntimeError('Multiple %r drivers found: %s' %
+                               (namespace,
+                                ','.join('%s:%s' % (e.module_name, e.attrs[0])
+                                         for e in self.extensions))
+                               )

stevedore/tests/test_driver.py

@@ -0,0 +1,17 @@
+"""Tests for stevedore.extension
+"""
+
+from stevedore import driver
+
+
+def test_detect_plugins():
+    em = driver.DriverManager('stevedore.test.extension', 't1')
+    names = sorted(em.names())
+    assert names == ['t1']
+
+
+def test_no_drivers():
+    try:
+        driver.DriverManager('stevedore.test.extension.none', 't1')
+    except RuntimeError as err:
+        assert "No 'stevedore.test.extension.none' driver found" == str(err)