Python pandas read_csv 中NULL当成NaN类型问题

1､使用read_csv读取数据null显示NaN

import pandas as pd
from io import StringIO

data = u'strings,numbers\nfoo,1\nbar,2\nnull,3'
print(pd.read_csv(StringIO(data)))

输出：

 strings  numbers
0     foo        1
1     bar        2
2     NaN        3

2､使用read_csv读取null数据显示为字符串null

import pandas as pd
from io import StringIO

data = u'strings,numbers\nfoo,1\nbar,2\nnull,3'
pd.read_csv(StringIO(data), converters={'strings' : str})
#或
pd.read_csv(StringIO(data), na_filter=False)
#或
df = pd.read_csv(io.StringIO(data), keep_default_na=False)

  strings  numbers
0     foo        1
1     bar        2
2    null        3