python - pandas series can't get index

Question

not sure what the problem is here... all i want is the first and only element in this series

>>> 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:Python27libsite-packagespandascoreseries.py", line 601, in __getitem__
    result = self.index.get_value(self, key)
  File "C:Python27libsite-packagespandascoreindexesase.py", line 2477, in get_value
    tz=getattr(series.dtype, 'tz', None))
  File "pandas\_libsindex.pyx", line 98, in pandas._libs.index.IndexEngine.get_value (pandas\_libsindex.c:4404)
  File "pandas\_libsindex.pyx", line 106, in pandas._libs.index.IndexEngine.get_value (pandas\_libsindex.c:4087)
  File "pandas\_libsindex.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libsindex.c:5126)
  File "pandas\_libshashtable_class_helper.pxi", line 759, in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libshashtable.c:14031)
  File "pandas\_libshashtable_class_helper.pxi", line 765, in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libshashtable.c:13975)
KeyError: 0L

why isn't that working? and how do get the first element?

Answer 1

When the index is integer, you cannot use positional indexers because the selection would be ambiguous (should it return based on label or position?). You need to either explicitly use a.iloc[0] or pass the label a[1].

The following works because the index type is object:

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

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

a[0]
Out: 1

But for integer index, things are different:

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