class AxisError(ValueError, IndexError):
    def __init__(self, axis, ndim=None, msg_prefix=None):
        if ndim is None and msg_prefix is None:
            # single-argument form (for backwards compatibility) sets just the message
            self._msg = axis
            self.axis = None
            self.ndim = None
        else:
            self._msg = msg_prefix
            self.axis = axis
            self.ndim = ndim

    def __str__(self):
        if self.axis is not None and self.ndim is not None:
            msg = "axis {} is out of bounds for array of dimension {}".format(
                self.axis, self.ndim)
            if self._msg is not None:
                msg = "{}: {}".format(self._msg, msg)
            return msg
        else:
            return self._msg