class Signal(object):
    """
    Base class for all signals

    Internal attributes:

        receivers
            { receriverkey (id) : weakref(receiver) }
    """

    def __init__(self, providing_args=None):
        """
        Create a new signal.

        providing_args
            A list of the arguments this signal can pass along in a send() call.
        """
        self.receivers = []
        if providing_args is None:
            providing_args = []
        self.providing_args = set(providing_args)