奇葩问题ValueError: math domain error，已解决！！！

　　在实现决策树的ID3算法时，编写了计算信息熵的函数，运行后遇到了如下的问题：

 　　对应处的代码为：

    @staticmethod
    def calc_ent(datasets):
        data_length = len(datasets)
        label_count = {}
        for i in range(data_length):
            label = datasets[i][-1]
            if label not in label_count:
                label_count[label] = 0
            else:
                label_count[label] += 1
        ent = -sum([(p / data_length) * log(p / data_length, 2) for p in label_count.values()])
        return ent

　　经过不断地试错与百度，最后解决了该问题，竟然是把else:去掉：

    @staticmethod
    def calc_ent(datasets):
        data_length = len(datasets)
        label_count = {}
        for i in range(data_length):
            label = datasets[i][-1]
            if label not in label_count:
                label_count[label] = 0
            label_count[label] += 1
        ent = -sum([(p / data_length) * log(p / data_length, 2) for p in label_count.values()])
        return ent

　　运行成功：

 　　根据报错提示，原因应该是数学域错误，不知道为什么删掉else:就成功了......