Python dataframe:复制索引列到一个新列(错误)

Question

我有一个数据框(第一列是索引列)(下面)。我想添加(创建)一个(最后)列，这是索引列的副本(下面期望的结果)。然而，我有错误(如下)。有解决办法吗?提前感谢

import pandas as pd
df1 = pd.DataFrame({"date": ['2021-3-22', '2021-3-23', '2021-3-24', '2021-3-25', '2021-3-26'],
"x": ['nan', 1, 'nan', 'nan', 'nan' ]})
df1.set_index('date', inplace=True)
df1
date        x
2021-3-22   nan
2021-3-23   1
2021-3-24   nan
2021-3-25   nan
2021-3-26   nan
df1['date1'] = df1['date'].copy()
df1
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~anaconda3libsite-packagespandascoreindexesbase.py in get_loc(self, key, method, tolerance)
   3360             try:
-> 3361                 return self._engine.get_loc(casted_key)
   3362             except KeyError as err:
~anaconda3libsite-packagespandas_libsindex.pyx in pandas._libs.index.IndexEngine.get_loc()
~anaconda3libsite-packagespandas_libsindex.pyx in pandas._libs.index.IndexEngine.get_loc()
pandas_libshashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
pandas_libshashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
KeyError: 'date'
The above exception was the direct cause of the following exception:
KeyError                                  Traceback (most recent call last)
C:Temp/ipykernel_10224/2516020320.py in <module>
----> 1 df1['date1'] = df1['date'].copy()
      2 df1
~anaconda3libsite-packagespandascoreframe.py in __getitem__(self, key)
   3456             if self.columns.nlevels > 1:
   3457                 return self._getitem_multilevel(key)
-> 3458             indexer = self.columns.get_loc(key)
   3459             if is_integer(indexer):
   3460                 indexer = [indexer]
~anaconda3libsite-packagespandascoreindexesbase.py in get_loc(self, key, method, tolerance)
   3361                 return self._engine.get_loc(casted_key)
   3362             except KeyError as err:
-> 3363                 raise KeyError(key) from err
   3364 
   3365         if is_scalar(key) and isna(key) and not self.hasnans:
KeyError: 'date'

The desired result is: 
    date        x   date1
0   2021-3-22   nan 2021-3-22
1   2021-3-23   1   2021-3-23
2   2021-3-24   nan 2021-3-24
3   2021-3-25   nan 2021-3-25
4   2021-3-26   nan 2021-3-26

提前感谢!

Answer 1

df1.index代替df1['date']:

df1['date1'] = df1.index.copy()
df1.reset_index()

输出:

    date        x   date1
0   2021-3-22   nan 2021-3-22
1   2021-3-23   1   2021-3-23
2   2021-3-24   nan 2021-3-24
3   2021-3-25   nan 2021-3-25
4   2021-3-26   nan 2021-3-26

Answer 2

为了保持date列，调整您的解决方案并将drop=False添加到set_index()中。

import pandas as pd
df1 = pd.DataFrame({"date": ['2021-3-22', '2021-3-23', '2021-3-24', '2021-3-25', '2021-3-26'],
"x": ['nan', 1, 'nan', 'nan', 'nan' ]})
df1.set_index('date', inplace=True, drop=False)
df1
>>>              date     x
date        
2021-3-22   2021-3-22   nan
2021-3-23   2021-3-23   1
2021-3-24   2021-3-24   nan
2021-3-25   2021-3-25   nan
2021-3-26   2021-3-26   nan

你可以使用df1['date1'] = df1['date'].copy()或者用df1.rename({'date':'date1'}, axis=1)将date重命名为date1。

df1 = df1.rename({'date':'date1'}, axis=1)
df1
>>>             date1     x
date        
2021-3-22   2021-3-22   nan
2021-3-23   2021-3-23   1
2021-3-24   2021-3-24   nan
2021-3-25   2021-3-25   nan
2021-3-26   2021-3-26   nan