Pandas新增合并列报错：ValueError: 值长度与索引长度不匹配

尝试在CSV数据集新增一列，将title、authors、publisher三列内容合并到新列中，编写了combine_features函数实现逻辑，但运行时报错：

ValueError: Length of values (1) does not match length of index (11123)

已尝试df.reset_index(inplace=True,drop=True)，问题依旧。

代码如下：

import numpy as np
import pandas as pd

df = pd.read_csv('books.csv', encoding='unicode_escape', error_bad_lines=False)

#List of columns to keep
columns =['title', 'authors', 'publisher']

#Function to combine the columns/features
def combine_features(data):
  features = []
  for i in range(0, data.shape[0]):
    features.append( data['title'][i] +' '+data['authors'][i]+' '+data['publisher'][i])
    return features

#Column to store the combined features
df['combined_features'] =combine_features(df)

#Show data
df

完整报错信息：

ValueError                                Traceback (most recent call last)
<ipython-input-24-40cc76d3cd85> in <module>
      1 #Create a column to store the combined features
----> 2 df['combined_features'] =combine_features(df)
      3 df

3 frames
/usr/local/lib/python3.8/dist-packages/pandas/core/frame.py in __setitem__(self, key, value)
   3610         else:
   3611             # set column
-> 3612             self._set_item(key, value)
   3613 
   3614     def _setitem_slice(self, key: slice, value):

/usr/local/lib/python3.8/dist-packages/pandas/core/frame.py in _set_item(self, key, value)
   3782         ensure homogeneity.
   3783         """
-> 3784         value = self._sanitize_column(value)
   3785 
   3786         if (

/usr/local/lib/python3.8/dist-packages/pandas/core/frame.py in _sanitize_column(self, value)
   4507 
   4508         if is_list_like(value):
-> 4509             com.require_length_match(value, self.index)
   4510         return sanitize_array(value, self.index, copy=True, allow_2d=True)
   4511 

/usr/local/lib/python3.8/dist-packages/pandas/core/common.py in require_length_match(data, index)
    529     """
    530     if len(data) != len(index):
-> 531         raise ValueError(
    532             "Length of values "
    533             f"({len(data)}) "

ValueError: Length of values (1) does not match length of index (11123)

错误原因 #

combine_features函数里的return features缩进错误，它被放在了for循环内部。这导致循环仅执行一次（添加第一行的合并内容）就直接返回列表，最终features只有1个元素，和DataFrame的11123行长度不匹配，触发报错。

修正后的代码 #

把return features移到for循环外面，确保遍历完所有行再返回完整列表：

import numpy as np
import pandas as pd

df = pd.read_csv('books.csv', encoding='unicode_escape', error_bad_lines=False)

#Function to combine the columns/features
def combine_features(data):
  features = []
  for i in range(0, data.shape[0]):
    features.append( data['title'][i] +' '+data['authors'][i]+' '+data['publisher'][i])
  # 将return移至循环外部
  return features

#Column to store the combined features
df['combined_features'] = combine_features(df)

#Show data
df

更高效的写法（推荐） #

用Pandas的向量操作替代循环，代码更简洁且运行效率更高：

import pandas as pd

df = pd.read_csv('books.csv', encoding='unicode_escape', error_bad_lines=False)

# 直接通过向量拼接合并三列
df['combined_features'] = df['title'] + ' ' + df['authors'] + ' ' + df['publisher']

# 查看结果
df

内容的提问来源于stack exchange，提问作者mudgey