title: get index column error from dataframe from
body: <p>I simply want to get index column.</p>
  
  <pre><code>import pandas as pd
  df1=pd.read_csv(path1, index_col='ID') 
  df1.head()
  
  
         VAR1  VAR2  VAR3  OUTCOME
  ID
  28677    28     1   0.0        0
  27170    59     1   0.0        1
  39245    65     1   0.0        1
  31880    19     1   0.0        0
  41441    24     1   0.0        1
  </code></pre>
  
  <p>I can get many columns like:</p>
  
  <pre><code>df1["VAR1"]
  
  ID
  28677     28
  27170     59
  39245     65
  31880     19
  41441     24
  31070     77
  39334     63
  ....
  38348     23
  38278     52
  28177     58
  </code></pre>
  
  <p>but, I cannot get index column:</p>
  
  <pre><code>&gt;&gt;&gt; df1["ID"]
  Traceback (most recent call last):
  File "C:\Anaconda3\lib\site-packages\pandas\indexes\base.py", line 2134, in get_loc
  return self._engine.get_loc(key)
  File "pandas\index.pyx", line 132, in pandas.index.IndexEngine.get_loc (pandas\index.c:4433)
  File "pandas\index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas\index.c:4279)
  File "pandas\src\hashtable_class_helper.pxi", line 732, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13742)
  File "pandas\src\hashtable_class_helper.pxi", line 740, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13696)
  KeyError: 'ID'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "C:\Anaconda3\lib\site-packages\pandas\core\frame.py", line 2059, in __getitem__
  return self._getitem_column(key)
  File "C:\Anaconda3\lib\site-packages\pandas\core\frame.py", line 2066, in _getitem_column
  return self._get_item_cache(key)
  File "C:\Anaconda3\lib\site-packages\pandas\core\generic.py", line 1386, in _get_item_cache
  values = self._data.get(item)
  File "C:\Anaconda3\lib\site-packages\pandas\core\internals.py", line 3543, in get
  loc = self.items.get_loc(item)
  File "C:\Anaconda3\lib\site-packages\pandas\indexes\base.py", line 2136, in get_loc
  return self._engine.get_loc(self._maybe_cast_indexer(key))
  File "pandas\index.pyx", line 132, in pandas.index.IndexEngine.get_loc (pandas\index.c:4433)
  File "pandas\index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas\index.c:4279)
  File "pandas\src\hashtable_class_helper.pxi", line 732, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13742)
  File "pandas\src\hashtable_class_helper.pxi", line 740, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13696)
  KeyError: 'ID'
  </code></pre>
  
  <p>I want to get the list of index column.
  how to do it?
  why I get error?</p>
  
  <p>If I want to merge two dataframe use the index column, how to do it?</p>