为什么这个分类Python dataframe列行为不正常

import pandas as pd
dfg = pd.DataFrame(['A+', 'A', 'A-', 'B+', 'B', 'B-', 'C+', 'C', 'C-', 'D+', 'D'],
                  index=['excellent', 'excellent', 'excellent', 'good', 'good', 'good', 'ok', 'ok', 'ok', 'poor', 'poor'])
dfg.rename(columns={0: 'Grades'}, inplace=True)

dfg['Grades'] = dfg['Grades'].astype('category',
                             categories=['D', 'D+', 'C-', 'C', 'C+', 'B-', 'B', 'B+', 'A-', 'A', 'A+'],
                             ordered=True)

grades=dfg['Grades']
for x in grades:
    print x,x>'C+'

查看我的结果:

A+ False
A False
A- False
B+ False
B False
B- False
C+ True
C False
C- True
D+ True
D True