相关疑难解决方法(0)

我从这段代码中得到了这个例外:

class Transaction:
    def __init__ (self):
        self.materials = {}

    def add_material (self, m):
        self.materials[m.type + m.purity] = m

    def serialize (self):
        ser_str = 'transaction_start\n'

        for k, m in self.materials:
            ser_str += m.serialize ()

        sert += 'transaction_end\n'
        return ser_str

该for行是抛出异常的行.该ms为Material对象.有人有什么想法吗？