python - 使用 pandas.read_csv 索引 DataFrame pandas

Question

我有一个这种格式的文件文本:

            Value_1     Value_2     Value_3     Value_4     Value_5
10  MSX 0.40748157  0.223845099 0.14417411  0.434948943 0.804686917
20  MSX 0.679787708 0.669394102 0.09966376  0.295964602 0.741914305
30  MSX 0.746162656 0.325330912 0.739320885 0.999178387 0.492413352
40  MSX 0.320922967 0.015966342 0.599784939 0.621426555 0.393276648
50  MSX 0.306445206 0.412270302 0.18820778  0.605985775 0.21907484
60  MSX 0.238848223 0.188215196 0.546380095 0.683780446 0.868291677
70  MSX 0.181441931 0.868270404 0.483950835 0.398810737 0.874501167
80  MSX 0.559737008 0.991398039 0.737921625 0.998737594 0.033864231
90  MSX 0.648728689 0.832948225 0.558207809 0.712151967 0.11245818
100 MSX 0.467005098 0.378185352 0.347756793 0.098410037 0.189492058

我用 pandas.read_csv 读取这个文件是这样的:

import pandas

db = "data.txt"

a = pandas.read_csv(db, sep='\t')

例如，我想获取第 40 行和列 Value_3 的值。

当我使用时:

a["Value_3"]

我有 Value_3 的所有列值:

>>> a["Value_3"]
    10   MSX    0.144174
    20   MSX    0.099664
    30   MSX    0.739321
    40   MSX    0.599785
    50   MSX    0.188208
    60   MSX    0.546380
    70   MSX    0.483951
    80   MSX    0.737922
    90   MSX    0.558208
    100  MSX    0.347757
    Name: Value_3

当我使用 ["40", "Value_3"] 时出现此错误消息:

Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    a["40","Value_3"]
  File "C:\Program Files (x86)\python27\lib\site-packages\pandas\core\frame.py", line 1704, in __getitem__
    return self._get_item_cache(key)
  File "C:\Program Files (x86)\python27\lib\site-packages\pandas\core\generic.py", line 474, in _get_item_cache
    values = self._data.get(item)
  File "C:\Program Files (x86)\python27\lib\site-packages\pandas\core\internals.py", line 827, in get
    _, block = self._find_block(item)
  File "C:\Program Files (x86)\python27\lib\site-packages\pandas\core\internals.py", line 941, in _find_block
    self._check_have(item)
  File "C:\Program Files (x86)\python27\lib\site-packages\pandas\core\internals.py", line 948, in _check_have
    raise KeyError('no item named %s' % str(item))
KeyError: "no item named ('40', 'Value_3')"

谁能帮帮我？

Answer 1

这里有很好的记录Advanced indexing with hierarchical index .

演示:

In [455]: df.loc[(slice(10,20), slice('MSX')), :]
Out[455]:
         Value_1   Value_2   Value_3   Value_4   Value_5
10 MSX  0.407482  0.223845  0.144174  0.434949  0.804687
20 MSX  0.679788  0.669394  0.099664  0.295965  0.741914