Add map_method function to managers

This allows to use map() directly over some extension method in a more
convenient way. It's a pattern we often need, so let's built it directly
in Stevedore.

stevedore/dispatch.py

@@ -73,6 +73,29 @@ class DispatchExtensionManager(EnabledExtensionManager):
                 self._invoke_one_plugin(response.append, func, e, args, kwds)
         return response
 
+    def map_method(self, filter_func, method_name, *args, **kwds):
+        """Iterate over the extensions invoking each one's object method called
+        `method_name` for any where filter_func() returns True.
+
+        This is equivalent of using :meth:`map` with func set to
+        `lambda x: x.obj.method_name()`
+        while being more convenient.
+
+        Exceptions raised from within the called method are propagated up
+        and processing stopped if self.propagate_map_exceptions is True,
+        otherwise they are logged and ignored.
+
+        .. versionadded:: 0.12
+
+        :param filter_func: Callable to test each extension.
+        :param method_name: The extension method name to call for each extension.
+        :param args: Variable arguments to pass to method
+        :param kwds: Keyword arguments to pass to method
+        :returns: List of values returned from methods
+        """
+        return self.map(filter_func, self._call_extension_method,
+                        method_name, *args, **kwds)
+
 
 class NameDispatchExtensionManager(DispatchExtensionManager):
     """Loads all plugins and filters on execution.
@@ -151,3 +174,26 @@ class NameDispatchExtensionManager(DispatchExtensionManager):
             else:
                 self._invoke_one_plugin(response.append, func, e, args, kwds)
         return response
+
+    def map_method(self, names, method_name, *args, **kwds):
+        """Iterate over the extensions invoking each one's object method called
+        `method_name` for any where the name is in the given list of names.
+
+        This is equivalent of using :meth:`map` with func set to
+        `lambda x: x.obj.method_name()`
+        while being more convenient.
+
+        Exceptions raised from within the called method are propagated up
+        and processing stopped if self.propagate_map_exceptions is True,
+        otherwise they are logged and ignored.
+
+        .. versionadded:: 0.12
+
+        :param names: List or set of name(s) of extension(s) to invoke.
+        :param method_name: The extension method name to call for each extension.
+        :param args: Variable arguments to pass to method
+        :param kwds: Keyword arguments to pass to method
+        :returns: List of values returned from methods
+        """
+        return self.map(names, self._call_extension_method,
+                        method_name, *args, **kwds)

stevedore/extension.py

@@ -140,6 +140,30 @@ class ExtensionManager(object):
             self._invoke_one_plugin(response.append, func, e, args, kwds)
         return response
 
+    @staticmethod
+    def _call_extension_method(extension, method_name, *args, **kwds):
+        return getattr(extension.obj, method_name)(*args, **kwds)
+
+    def map_method(self, method_name, *args, **kwds):
+        """Iterate over the extensions invoking each one's object method called `method_name`.
+
+        This is equivalent of using :meth:`map` with func set to
+        `lambda x: x.obj.method_name()`
+        while being more convenient.
+
+        Exceptions raised from within the called method are propagated up
+        and processing stopped if self.propagate_map_exceptions is True,
+        otherwise they are logged and ignored.
+
+        .. versionadded:: 0.12
+
+        :param method_name: The extension method name to call for each extension.
+        :param args: Variable arguments to pass to method
+        :param kwds: Keyword arguments to pass to method
+        :returns: List of values returned from methods
+        """
+        return self.map(self._call_extension_method, method_name, *args, **kwds)
+
     def _invoke_one_plugin(self, response_callback, func, e, args, kwds):
         try:
             response_callback(func(e, *args, **kwds))

stevedore/tests/test_dispatch.py

@@ -1,11 +1,12 @@
 from stevedore import dispatch
 
 
-def test_dispatch():
-
-    def check_dispatch(ep, *args, **kwds):
+def check_dispatch(ep, *args, **kwds):
     return ep.name == 't2'
 
+
+def test_dispatch():
+
     def invoke(ep, *args, **kwds):
         return (ep.name, args, kwds)
 
@@ -28,6 +29,20 @@ def test_dispatch():
     assert results == expected
 
 
+def test_dispatch_map_method():
+    em = dispatch.DispatchExtensionManager(
+        'stevedore.test.extension',
+        lambda *args, **kwds: True,
+        invoke_on_load=True,
+        invoke_args=('a',),
+        invoke_kwds={'b': 'B'},
+    )
+
+    results = em.map_method(check_dispatch, 'get_args_and_data',
+                            'first')
+    assert results == [(('a',), {'b': 'B'}, 'first')]
+
+
 def test_name_dispatch():
 
     def invoke(ep, *args, **kwds):
@@ -72,3 +87,16 @@ def test_name_dispatch_ignore_missing():
                      )
     expected = [('t1', ('first',), {'named': 'named value'})]
     assert results == expected
+
+def test_name_dispatch_map_method():
+    em = dispatch.NameDispatchExtensionManager(
+        'stevedore.test.extension',
+        lambda *args, **kwds: True,
+        invoke_on_load=True,
+        invoke_args=('a',),
+        invoke_kwds={'b': 'B'},
+    )
+
+    results = em.map_method(['t3', 't1'], 'get_args_and_data',
+                            'first')
+    assert results == [(('a',), {'b': 'B'}, 'first')]

stevedore/tests/test_extension.py

@@ -11,6 +11,9 @@ class FauxExtension(object):
         self.args = args
         self.kwds = kwds
 
+    def get_args_and_data(self, data):
+        return self.args, self.kwds, data
+
 
 def test_detect_plugins():
     em = extension.ExtensionManager('stevedore.test.extension')
@@ -155,3 +158,12 @@ def test_map_errors_when_no_plugins():
         em.map(mapped, 1, 2, a='A', b='B')
     except RuntimeError as err:
         assert 'No stevedore.test.extension.none extensions found' == str(err)
+
+
+def test_map_method():
+    em = extension.ExtensionManager('stevedore.test.extension',
+                                    invoke_on_load=True,
+                                    )
+
+    result = em.map_method('get_args_and_data', 42)
+    assert set(r[2] for r in result) == set([42])