Add ExtensionManager.__getitem__

Allow extensions to be accessed directly by name.

Fixes issue #15

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

docs/source/history.rst

@@ -12,6 +12,9 @@ dev
 - Change the
   :class:`~stevedore.dispatch.NamedDispatchExtensionManager` to ignore
   missing extensions (:issue:`14`).
+- Add ``__getitem__`` to
+  :class:`~stevedore.extension.ExtensionManager` for looking up
+  individual plugins by name (:issue:`15`).
 
 0.8
 

stevedore/extension.py

@@ -55,6 +55,7 @@ class ExtensionManager(object):
         self.extensions = self._load_plugins(invoke_on_load,
                                              invoke_args,
                                              invoke_kwds)
+        self._extensions_by_name = None
 
     ENTRY_POINT_CACHE = {}
 
@@ -93,6 +94,9 @@ class ExtensionManager(object):
 
     def names(self):
         "Returns the names of the discovered extensions"
+        # We want to return the names of the extensions in the order
+        # they would be used by map(), since some subclasses change
+        # that order.
         return [e.name for e in self.extensions]
 
     def map(self, func, *args, **kwds):
@@ -133,3 +137,17 @@ class ExtensionManager(object):
 
     def __iter__(self):
         return iter(self.extensions)
+
+    def __getitem__(self, name):
+        """Return the named extension.
+
+        Accessing an ExtensionManager as a dictionary (``em['name']``)
+        produces the :class:`Extension` instance with the
+        specified name.
+        """
+        if self._extensions_by_name is None:
+            d = {}
+            for e in self.extensions:
+                d[e.name] = e
+            self._extensions_by_name = d
+        return self._extensions_by_name[name]

stevedore/hook.py

@@ -23,6 +23,7 @@ class HookManager(NamedExtensionManager):
 
     def __init__(self, namespace, name,
                  invoke_on_load=False, invoke_args=(), invoke_kwds={}):
+        self._name = name
         super(HookManager, self).__init__(
             namespace,
             [name],
@@ -30,3 +31,14 @@ class HookManager(NamedExtensionManager):
             invoke_args=invoke_args,
             invoke_kwds=invoke_kwds,
         )
+
+    def __getitem__(self, name):
+        """Return the named extensions.
+
+        Accessing a HookManager as a dictionary (``em['name']``)
+        produces a list of the :class:`Extension` instance(s) with the
+        specified name, in the order they would be invoked by map().
+        """
+        if name != self._name:
+            raise KeyError(name)
+        return self.extensions

stevedore/tests/test_extension.py

@@ -18,6 +18,22 @@ def test_detect_plugins():
     assert names == ['t1', 't2']
 
 
+def test_get_by_name():
+    em = extension.ExtensionManager('stevedore.test.extension')
+    e = em['t1']
+    assert e.name == 't1'
+
+
+def test_get_by_name_missing():
+    em = extension.ExtensionManager('stevedore.test.extension')
+    try:
+        em['t3']
+    except KeyError:
+        pass
+    else:
+        assert False, 'Failed to raise KeyError'
+
+
 def test_load_multiple_times_entry_points():
     # We expect to get the same EntryPoint object because we save them
     # in the cache.

stevedore/tests/test_hook.py

@@ -11,3 +11,33 @@ def test_hook():
     )
     assert len(em.extensions) == 1
     assert em.names() == ['t1']
+
+
+def test_get_by_name():
+    em = hook.HookManager(
+        'stevedore.test.extension',
+        't1',
+        invoke_on_load=True,
+        invoke_args=('a',),
+        invoke_kwds={'b': 'B'},
+    )
+    e_list = em['t1']
+    assert len(e_list) == 1
+    e = e_list[0]
+    assert e.name == 't1'
+
+
+def test_get_by_name_missing():
+    em = hook.HookManager(
+        'stevedore.test.extension',
+        't1',
+        invoke_on_load=True,
+        invoke_args=('a',),
+        invoke_kwds={'b': 'B'},
+    )
+    try:
+        em['t2']
+    except KeyError:
+        pass
+    else:
+        assert False, 'Failed to raise KeyError'