calculate xml expression

from xml.dom.minidom import parseString

dom = parseString("<expression><operation type=\"+\"><operand>2</operand><operand><operation type=\"*\"><operand>3</operand><operand>4</operand></operation></operand></operation></expression>")

def calc_xml_expr (expr_node) :
    if expr_node.nodeName == "operation" :
        res = 0
        op_code = expr_node.getAttribute("type")
        oprndElems = expr_node.getElementsByTagName("operand")
        if op_code == "+" :
            return calc_xml_expr(oprndElems[0]) + calc_xml_expr(oprndElems[1])
        elif op_code == "*" :
            return calc_xml_expr(oprndElems[0]) * calc_xml_expr(oprndElems[1])
        
    if expr_node.nodeName == "operand" :
        child_node = expr_node.childNodes[0]
        if child_node.nodeType == child_node.TEXT_NODE :
            return int(child_node.nodeValue)
        else:
            return calc_xml_expr(child_node)
   
print calc_xml_expr(dom.getElementsByTagName("expression")[0].getElementsByTagName("operation")[0])

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