ValueError：不能将现有列的名称用于指示符列

Posted 2023-03-29

【中文标题】ValueError：不能将现有列的名称用于指示符列

【英文标题】：ValueError: Cannot use name of an existing column for indicator column

【发布时间】：2018-07-18 01:38:39

【问题描述】：

我需要解决一个问题，我将有一个数据框，比如 df，名称和年龄，我需要在 for 循环中生成另一个名称和性别的数据框，我需要合并 for 中生成的数据框与 df 循环以获取 df 中的性别。所以我在解决我的问题之前尝试了下面的代码

import pandas as pd
d = 'Age': [45, 38], 'Name': ['John', 'Emily']
df = pd.DataFrame(data=d)
d1='Gender':['M'],'Name':['John']
df1=pd.DataFrame(data=d1)

df3 = df.merge(df1, on=['Name'], how='left', indicator=True)
df3

d2='Gender':['F'],'Name':['Emily']
df4=pd.DataFrame(data=d2)
df5=df3.merge(df4, on=['Name'], how='left', indicator=True)

我在运行最后一行时遇到以下错误。

 "Cannot use name of an existing column for indicator column")

ValueError: Cannot use name of an existing column for indicator column

你能建议我如何在 python 3.x 中解决这个问题吗？

【参考方案1】：

有更好的方法来完成你想做的事情（正如另一个人回答的那样）。但要了解您收到错误的原因，请阅读以下内容。

因为您进行了一次合并，所以您现在在 df3.xml 中有一个名为 _merge 的列。当你再次合并时，你不能再创建另一个_merge。

顺便说一句，为了将来参考，现在你有indicator=True，但你也可以传入一个字符串，例如indicator='exists'，然后你的“指示”你如何加入的新列将被称为exists，你可以通过df5['exists']选择它

查看这个简单的示例并在 repl 中浏览它

>>> df1
  col1 col2
0    a    b
1    b    c
2    d    e
>>> df2
  col1 col2
0    a    b
1    b    c
>>> df1.merge(df2, on='col1', how='left', indicator=True)
  col1 col2_x col2_y     _merge
0    a      b      b       both
1    b      c      c       both
2    d      e    NaN  left_only
>>> df3 = df1.merge(df2, on='col1', how='left', indicator=True)
>>> df4 = pd.DataFrame([['d', 'e']], columns=['col1', 'col2'])
>>> df3.merge(df4, on='col1', how='left', indicator=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/devtools/uat/anaconda4321/lib/python3.6/site-packages/pandas/core/frame.py", line 4722, in merge
copy=copy, indicator=indicator)
  File "/usr/local/devtools/uat/anaconda4321/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 54, in merge
return op.get_result()
  File "/usr/local/devtools/uat/anaconda4321/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 567, in get_result
self.left, self.right)
  File "/usr/local/devtools/uat/anaconda4321/lib/python3.6/site-packages/pandas/core/reshape/merge.py", line 605, in _indicator_pre_merge
"Cannot use name of an existing column for indicator column")
ValueError: Cannot use name of an existing column for indicator column
>>> df3.merge(df4, on='col1', how='left', indicator='exists')
  col1 col2_x col2_y     _merge col2     exists
0    a      b      b       both  NaN  left_only
1    b      c      c       both  NaN  left_only
2    d      e    NaN  left_only    e       both

【参考方案2】：

我将采取与其他人想象的不同的方式。我会用map()

# merging both gender dataframes together for convenience 
gender = pd.concat([df1,df4])

# creating a column the same as 'Name' but calling it gender
df['Gender'] = df['Name']

# creating a dictionary with the name as the key, and gender as value
gender_dict = gender.set_index('Name')['Gender'].to_dict()
# output as 'Emily': 'F', 'John': 'M'

# remapping the name in place of the gender
df['Gender'] = df['Gender'].map(gender_dict)


    Age Name    Gender
0   45  John    M
1   38  Emily   F