pandas 按名称引用索引列 [英] Pandas referencing index column by name

问题描述

我正在使用pandas pandas-0.23.4-cp36-cp36m-manylinux1_x86_64.whl，并且我注意到将列设置为索引列时，无法再按名称引用它. 将其设置为索引后，有什么方法可以引用该列? 下面的代码引发KeyError.

I'm using pandas pandas-0.23.4-cp36-cp36m-manylinux1_x86_64.whl and I noticed that when you set a column as an index column, you can no longer reference it by name. Is there any way to reference the column after you've set it as an index? The code below raises 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tscol成为索引之前和之后，您可以在此处看到数据框:

Here you can see the dataframe, before and after the tscol becomes an index:

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

给我这个例外情况

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'

推荐答案

在设置索引以将其保留为DataFrame中的列时，您可以传递参数drop=False:

You can pass the argument drop=False when setting the index to keep it as a column in the DataFrame:

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

这篇关于 pandas 按名称引用索引列的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！