python - Pandas 系列无法获得索引

Question

不知道这里有什么问题......我想要的是这个系列中的第一个也是唯一的元素

>>> a
1    0-5fffd6b57084003b1b582ff1e56855a6!1-AB8769635...
Name: id, dtype: object

>>> len (a)
1

>>> type(a)
<class 'pandas.core.series.Series'>

>>> a[0]

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    a[0]
  File "C:\Python27\lib\site-packages\pandas\core\series.py", line 601, in __getitem__
    result = self.index.get_value(self, key)
  File "C:\Python27\lib\site-packages\pandas\core\indexes\base.py", line 2477, in get_value
    tz=getattr(series.dtype, 'tz', None))
  File "pandas\_libs\index.pyx", line 98, in pandas._libs.index.IndexEngine.get_value (pandas\_libs\index.c:4404)
  File "pandas\_libs\index.pyx", line 106, in pandas._libs.index.IndexEngine.get_value (pandas\_libs\index.c:4087)
  File "pandas\_libs\index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5126)
  File "pandas\_libs\hashtable_class_helper.pxi", line 759, in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:14031)
  File "pandas\_libs\hashtable_class_helper.pxi", line 765, in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:13975)
KeyError: 0L

为什么不起作用？以及如何获取第一个元素？

Answer 1

当索引为整数时，您不能使用位置索引器，因为选择会产生歧义(它应该根据标签还是位置返回？)。您需要显式使用 a.iloc[0] 或传递标签 a[1]。

以下内容有效，因为索引类型是对象:

a = pd.Series([1, 2, 3], index=['a', 'b', 'c'])

a
Out: 
a    1
b    2
c    3
dtype: int64

a[0]
Out: 1

但是对于整数索引，情况就不同了:

a = pd.Series([1, 2, 3], index=[2, 3, 4])

a[2]  # returns the first entry - label based
Out: 1

a[1]  # raises a KeyError
KeyError: 1