<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def export_ownertrust(cls, trustdb=None):
 """Export ownertrust to a trustdb file. If there is already a file named :file:`trustdb.gpg` in the current GnuPG homedir, it will be renamed to :file:`trustdb.gpg.bak`. :param string trustdb: The path to the trustdb.gpg file. If not given, defaults to ``'trustdb.gpg'`` in the current GnuPG homedir. """

    if trustdb is None:
        trustdb = os.path.join(cls.homedir, 'trustdb.gpg')

    try:
        os.rename(trustdb, trustdb + '.bak')
    except (OSError, IOError) as err:
        log.debug(str(err))

    export_proc = cls._open_subprocess(['--export-ownertrust'])
    tdb = open(trustdb, 'wb')
    _util._threaded_copy_data(export_proc.stdout, tdb)
    export_proc.wait()