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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 update_ports_tree(ports_tree):
    '''
    Updates the ports tree, either the default or the `ports_tree` specified

    CLI Example:

    .. code-block:: bash

        salt '*' poudriere.update_ports_tree staging
    '''
    _check_config_exists()
    if ports_tree:
        cmd = 'poudriere ports -u -p {0}'.format(ports_tree)
    else:
        cmd = 'poudriere ports -u'
    ret = __salt__['cmd.run'](cmd)
    return ret