import importlib

PLUGIN_NAMESPACE = 'plugs'

_classes = []
_instances = {}


class PapersPlugin(object):
    """The base class for all plugins. Plugins provide
    functionality by defining a subclass of PapersPlugin and overriding
    the abstract methods defined here.
    """

    name = None

    def get_commands(self, subparsers):
        """Populates the parser with plugins specific command.
        Returns iterable of pairs (command name, command function to call).
        """
        return []

    @classmethod
    def get_instance(cls):
        if cls in _instances:
            return _instances[cls]
        else:
            raise RuntimeError("{} instance not created".format(cls.__name__))


def load_plugins(ui, names):
    """Imports the modules for a sequence of plugin names. Each name
    must be the name of a Python module under the "PLUGIN_NAMESPACE" namespace
    package in sys.path; the module indicated should contain the
    PapersPlugin subclasses desired.
    """
    for name in names:
        modname = '%s.%s.%s.%s' % ('papers', PLUGIN_NAMESPACE, name, name)
        try:
            namespace = importlib.import_module(modname)
        except ImportError as exc:
            # Again, this is hacky:
            if exc.args[0].endswith(' ' + name):
                ui.warning('plugin {} not found'.format(name))
            else:
                raise
        else:
            for obj in namespace.__dict__.values():
                if isinstance(obj, type) and issubclass(obj, PapersPlugin) \
                        and obj != PapersPlugin:
                    _classes.append(obj)
                    _instances[obj] = obj()


def get_plugins():
    return _instances