Allow flexible selector syntax.

    Returns:
        dict

def _select_helper(args, kwargs):
    """ Allow flexible selector syntax.

    Returns:
        dict

    """
    if len(args) > 1:
        raise TypeError("select accepts at most ONE positional argument.")

    if len(args) > 0 and len(kwargs) > 0:
        raise TypeError("select accepts EITHER a positional argument, OR keyword arguments (not both).")

    if len(args) == 0 and len(kwargs) == 0:
        raise TypeError("select requires EITHER a positional argument, OR keyword arguments.")

    if args:
        arg = args[0]
        if isinstance(arg, dict):
            selector = arg
        elif isinstance(arg, string_types):
            selector = dict(name=arg)
        elif isinstance(arg, type) and issubclass(arg, Model):
            selector = {"type": arg}
        else:
            raise TypeError("selector must be a dictionary, string or plot object.")

    elif 'selector' in kwargs:
        if len(kwargs) == 1:
            selector = kwargs['selector']
        else:
            raise TypeError("when passing 'selector' keyword arg, not other keyword args may be present")

    else:
        selector = kwargs

    return selector