precision_dic =
{51: 0.9375, 25: 0.9305555555555556, -1: 0.7619047619047619, 29: 0.9473684210526315, 6: 0.8108108108108109}
precision_dic.items 是一个list,每个list是一个tuple
然后对这个list进行排序
sorted(precision_dic.items(), key=lambda x:x[1])