Question

import pandas as pd

df_run = pd.read_csv('UserEventSummary.csv')
df_run.accountId[0] = 'first-' + str(df_run.accountId[0])

第三行给我这个错误：

/home/ec2-user/anaconda3/envs/python3/lib/python3.6/site-packages/ipykernel/__main__.py:1: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
if __name__ == '__main__':
/home/ec2-user/anaconda3/envs/python3/lib/python3.6/site-packages/pandas/core/indexing.py:194: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self._setitem_with_indexer(indexer, value)

我已经阅读了有关文档，并且对何时发生有了很好的了解，但不确定为什么我会在这里找到它。我不觉得我做错了什么或危险的事！？

我猜有更好的（更正确的）方法吗？

Answer 1

Pandas具有为可靠访问和设置标量而设计的特定方法。要通过标签进行标量设置，请使用at。要通过整数位置索引进行标量设置，请使用iat：

df_run['accountId'].iat[0] = f'first-{df_run["accountId"].iat[0]}'

就像这里一样，iat可以安全地用于设置和访问，从而避免出现切片警告。

切片警告的Python数据框副本

1 个答案: