def __init__(self, cbs):
        """
        Args:
            cbs(list): a list of :class:`Callback` instances.
        """
        # check type
        for cb in cbs:
            assert isinstance(cb, Callback), cb.__class__
        self.cbs = cbs

        Args:
            cbs(list): a list of :class:`Callback` instances.
        
tensorpack/callbacks/group.py