def like_prefix(value, start='%'):
    """
    gets a copy of string with `%` or couple of `_` values attached to beginning.

    it is to be used in like operator.

    :param str value: value to be processed.

    :param str start: start place holder to be prefixed.
                      it could be `%` or couple of `_` values
                      for exact matching.
                      defaults to `%` if not provided.

    :rtype: str
    """

    if value is None:
        return None

    return '{start}{value}'.format(start=start, value=value)