从pandas.core.series.Series提取数据时出现KeyError

Question

在以下ipython3会话中，我读取了格式不同的表，并对其中一列中的值求和：

In [278]: F = pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/feature_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", skip
     ...: rows=2, usecols=[6]).sum()

In [279]: S = pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", us
     ...: ecols=[6], header=None).sum()

In [280]: S
Out[280]: 
6    3551266
dtype: int64

In [281]: F
Out[281]: 
72    3164181
dtype: int64

In [282]: type(F)
Out[282]: pandas.core.series.Series

In [283]: type(S)
Out[283]: pandas.core.series.Series

In [284]: F[0]
Out[284]: 3164181

In [285]: S[0]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-285-5a4339994a41> in <module>()
----> 1 S[0]

/home/bli/.local/lib/python3.6/site-packages/pandas/core/series.py in __getitem__(self, key)
    601             result = self.index.get_value(self, key)
    602 
--> 603             if not is_scalar(result):
    604                 if is_list_like(result) and not isinstance(result, Series):
    605 

/home/bli/.local/lib/python3.6/site-packages/pandas/indexes/base.py in get_value(self, series, key)

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3323)()

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3026)()

pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4009)()

pandas/src/hashtable_class_helper.pxi in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8146)()

pandas/src/hashtable_class_helper.pxi in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8090)()

KeyError: 0

如果F和S对象是由相似的操作（sum）产生并且属于相同的类型（pandas.core.series.Series），它们为什么会有不同的行为？

提取我想要的值（一列的总和）的正确方法是什么？

编辑：尝试解决方案：

In [297]: F["72"]
Out[297]: 3164181

In [298]: S["6"]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4009)()

pandas/src/hashtable_class_helper.pxi in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8125)()

TypeError: an integer is required

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-298-0127424036a0> in <module>()
----> 1 S["6"]

/home/bli/.local/lib/python3.6/site-packages/pandas/core/series.py in __getitem__(self, key)
    601             result = self.index.get_value(self, key)
    602 
--> 603             if not is_scalar(result):
    604                 if is_list_like(result) and not isinstance(result, Series):
    605 

/home/bli/.local/lib/python3.6/site-packages/pandas/indexes/base.py in get_value(self, series, key)

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3323)()

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3026)()

pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4075)()

KeyError: '6'

进一步调查：

In [306]: print(S.index)
Int64Index([6], dtype='int64')

In [307]: print(F.index)
Index(['72'], dtype='object')

In [308]: S[6]
Out[308]: 3551266

因此，这两个对象最终具有不同类型的索引。这种行为让我想起了...

看来这header=None导致了用数字为索引的列S，而没有与header=None组合的skiprows=2结果是从第三行读取的数据生成了索引。（这揭示了我解析熊猫数据的方式中的一个错误……）

Answer 1

我认为您需要：

#select first value of one element series
f = F.iat[0]
#alternative 
#f = F.iloc[0]

要么：

#convert to numpy array and select first value
f = F.values[0]

要么：

f = F.item()

而且我认为您会出错，因为没有索引值0。

正如IanS所评论的那样，应该按索引值6和选择72：

f = F[72] 
#f = f.loc[72]

s = S[6]
#s = S.loc[6]

样品：

F = pd.Series([3164181], index=[72])

f = F[72] 
print (f)
3164181

print (F.index)
Int64Index([72], dtype='int64')

print (F.index.tolist())
[72]

f = F[0] 
print (f)

KeyError：0

您在中获得一个整数索引S，因为参数header=None-pandas添加了默认索引（0,1,...）。对于F使用的6th列称为'72'-它是字符串。有区别。