'Return the directory paths of plugins within ``self.directories``.
Paths are considered a plugin path if they pass ``self.is_plugin``.
:rtype:                 string generator
:returns:               paths of valid plugins'

def find_plugins(self):

    for directory in self.directories:
        for plugin_dir in sorted(os.listdir(directory)):
            plugin_path = os.path.join(directory, plugin_dir)
            if self.is_plugin(plugin_path):
                (yield plugin_path)
