In mako, call the function %
                %for l in get_lines(o.order_line):
OR 
            <% lines = get_lines(o.order_line)

In py file, 

class sale_order_report(report_sxw.rml_parse):
    def __init__(self, cr, uid, name, context):
        super(grid_report, self).__init__(cr, uid, name, context=context)
        self.localcontext.update({
            'time': time,
            'get_lines': self._get_lines,
        })