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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def open(filename, flag='c', protocol=None, writeback=False, maxsize=DEFAULT_MAXSIZE, timeout=DEFAULT_TIMEOUT):
 """Open a database file as a persistent dictionary. The persistent dictionary file is opened using :func:`dbm.open`, so performance will depend on which :mod:`dbm` modules are installed. :func:`open` chooses to open a :class:`Shelf <shelve.Shelf>`, :class:`LRUShelf`, :class:`TimeoutShelf`, or :class:`LRUTimeoutShelf` depending on the values of keyword arguments *maxsize* and *timeout*. A :data:`None` value for *maxsize* and *timeout* will disable the LRU cache management and automatic data timeout features respectively. :param filename: The base filename for the underlying database that is passed to :func:`dbm.open`. :param flag: The flag to pass to :func:`dbm.open`. :param protocol: The pickle protocol to pass to :func:`pickle.dump`. :param writeback: Whether or not to write back all accessed entries on :meth:`Shelf.sync <shelve.Shelf.sync>` and :meth:`Shelf.close <shelve.Shelf.close>` :type writeback: bool :param maxsize: The maximum size the container is allowed to grow to. ``0`` means that no size limit is enforced. :data:`None` means that LRU cache management is disabled. :type maxsize: integer or :data:`None` :param timeout: The default timeout value for data (in seconds). ``0`` means that the data never expires. :data:`None` means that automatic timeout features will be disabled. :type timeout: integer or :data:`None` :return: A shelf :rtype: :class:`~shelve.Shelf`, :class:`LRUShelf`, :class:`TimeoutShelf`, or :class:`LRUTimeoutShelf` """

    import dbm
    dict = dbm.open(filename, flag)
    if maxsize is None and timeout is None:
        return Shelf(dict, protocol, writeback)
    elif maxsize is None:
        return TimeoutShelf(dict, protocol, writeback, timeout=timeout)
    elif timeout is None:
        return LRUShelf(dict, protocol, writeback, maxsize=maxsize)
    return LRUTimeoutShelf(dict, protocol, writeback, timeout=timeout,
                           maxsize=maxsize)