Namespace plugin implementation

* Creates pelican.plugins
* Moves plugin related code under pelican.plugins
  * pelican.plugins.signals is now the location for signals, pelican.signals is kept
    for backwards compatibility
  * pelican.plugins._utils contains necessary bits for plugin discovery and loading.
    Logic from Pelican class is moved here. Pelican class now just asks for plugins
    and registers them
* Contains tests for old and new plugin loading

pelican/plugins/_utils.py

@@ -0,0 +1,77 @@
+import importlib
+import logging
+import pkgutil
+import sys
+
+import six
+
+
+logger = logging.getLogger(__name__)
+
+
+def iter_namespace(ns_pkg):
+    # Specifying the second argument (prefix) to iter_modules makes the
+    # returned name an absolute name instead of a relative one. This allows
+    # import_module to work without having to do additional modification to
+    # the name.
+    return pkgutil.iter_modules(ns_pkg.__path__, ns_pkg.__name__ + ".")
+
+
+def get_namespace_plugins(ns_pkg=None):
+    if ns_pkg is None:
+        import pelican.plugins as ns_pkg
+
+    return {
+        name: importlib.import_module(name)
+        for finder, name, ispkg
+        in iter_namespace(ns_pkg)
+        if ispkg
+    }
+
+
+def list_plugins(ns_pkg=None):
+    from pelican.log import init as init_logging
+    init_logging(logging.INFO)
+    ns_plugins = get_namespace_plugins(ns_pkg)
+    if ns_plugins:
+        logger.info('Plugins found:\n' + '\n'.join(ns_plugins))
+    else:
+        logger.info('No plugins are installed')
+
+
+def load_plugins(settings):
+    logger.debug('Finding namespace plugins')
+    namespace_plugins = get_namespace_plugins()
+    if namespace_plugins:
+        logger.debug('Namespace plugins found:\n' +
+                     '\n'.join(namespace_plugins))
+    plugins = []
+    if settings.get('PLUGINS') is not None:
+        _sys_path = sys.path[:]
+
+        for path in settings.get('PLUGIN_PATHS', []):
+            sys.path.insert(0, path)
+
+        for plugin in settings['PLUGINS']:
+            if isinstance(plugin, six.string_types):
+                logger.debug('Loading plugin `%s`', plugin)
+                # try to find in namespace plugins
+                if plugin in namespace_plugins:
+                    plugin = namespace_plugins[plugin]
+                elif 'pelican.plugins.{}'.format(plugin) in namespace_plugins:
+                    plugin = namespace_plugins['pelican.plugins.{}'.format(
+                        plugin)]
+                # try to import it
+                else:
+                    try:
+                        plugin = __import__(plugin, globals(), locals(),
+                                            str('module'))
+                    except ImportError as e:
+                        logger.error('Cannot load plugin `%s`\n%s', plugin, e)
+                        continue
+            plugins.append(plugin)
+        sys.path = _sys_path
+    else:
+        plugins = list(namespace_plugins.values())
+
+    return plugins