def simple_slice(value):
    """
    >>> simple_slice('2:5')
    (2, 5)
    >>> simple_slice('0:None')
    (0, None)
    """
    try:
        start, stop = value.split(':')
        start = ast.literal_eval(start)
        stop = ast.literal_eval(stop)
        if start is not None and stop is not None:
            assert start < stop
    except Exception:
        raise ValueError('invalid slice: %s' % value)
    return (start, stop)
>>> simple_slice('2:5')
    (2, 5)
    >>> simple_slice('0:None')
    (0, None)