python - Pandas 按名称引用索引列

Question

我正在使用 pandas pandas-0.23.4-cp36-cp36m-manylinux1_x86_64.whl 并且我注意到当您将列设置为索引列时，您无法再通过名称引用它.将其设置为索引后，是否有任何方法可以引用该列？下面的代码引发了 KeyError。

import pandas as pd
from datetime import datetime, timedelta
df = pd.DataFrame()

one_month = datetime.now() - timedelta(days=30)
ts_index = pd.date_range(one_month, periods=30, freq='1D')

df.insert(0, 'tscol', ts_index)
df.insert(1, 'value', 1.0)

print(df.head())

# set the timeseries column as the index.
df.set_index('tscol', inplace=True)

print(df.head())

for index, row in df.iterrows():
    print(row['tscol'])
    break

在这里你可以看到数据帧，在 tscol 成为索引之前和之后:

之前

                       tscol  value
0 2018-08-19 10:53:32.412154    1.0
1 2018-08-20 10:53:32.412154    1.0
2 2018-08-21 10:53:32.412154    1.0
3 2018-08-22 10:53:32.412154    1.0
4 2018-08-23 10:53:32.412154    1.0

之后

                            value
tscol                            
2018-08-19 10:53:32.412154    1.0
2018-08-20 10:53:32.412154    1.0
2018-08-21 10:53:32.412154    1.0
2018-08-22 10:53:32.412154    1.0
2018-08-23 10:53:32.412154    1.0

给我这个异常(exception)

Traceback (most recent call last):
  File "/home/ben/.local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3124, in get_value
    return libindex.get_value_box(s, key)
  File "pandas/_libs/index.pyx", line 55, in pandas._libs.index.get_value_box
  File "pandas/_libs/index.pyx", line 63, in pandas._libs.index.get_value_box
TypeError: 'str' object cannot be interpreted as an integer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "index_by_name.py", line 24, in <module>
    print(row['tscol'])
  File "/home/ben/.local/lib/python3.6/site-packages/pandas/core/series.py", line 767, in __getitem__
    result = self.index.get_value(self, key)
  File "/home/ben/.local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3132, in get_value
    raise e1
  File "/home/ben/.local/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3118, in get_value
    tz=getattr(series.dtype, 'tz', None))
  File "pandas/_libs/index.pyx", line 106, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 114, in pandas._libs.index.IndexEngine.get_value
  File "pandas/_libs/index.pyx", line 162, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 1492, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1500, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'tscol'

Answer 1

在设置索引时，您可以传递参数 drop=False 以将其保留为 DataFrame 中的一列:

df.set_index('tscol', inplace=True, drop=False)