即使使用loc(?)，也可以使用SettingWithCopyWarning [英] SettingWithCopyWarning, even when using loc (?)

问题描述

在我不希望出现的情况下，我得到SettingWithCopyWarning错误:

I get SettingWithCopyWarning errors in cases where I would not expect them:

N.In <38>: # Column B does not exist yet
N.In <39>: df['B'] = df['A']/25
N.In <40>: df['B'] = df['A']/50

/Users/josh/anaconda/envs/py27/lib/python2.7/site-packages/pandas/core/indexing.py:389: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  self.obj[item] = s

和

N.In <41>: df.loc[:,'B'] = df['A']/50

/Users/josh/anaconda/envs/py27/lib/python2.7/site-packages/pandas/core/indexing.py:389: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  self.obj[item] = s

为什么在情况1和2中会发生这种情况?

Why does it happen in case 1 and 2?

推荐答案

在情况1中，df['A']创建了df的副本.正如 Pandas文档所述，链接时可能会导致意外结果，从而引发警告.情况2看起来正确，但可能会出现误报:

In case 1, df['A'] creates a copy of df. As explained by the Pandas documentation, this can lead to unexpected results when chaining, thus a warning is raised. Case 2 looks correct, but false positives are possible:

警告:链接的分配警告/异常旨在 通知用户可能无效的分配.可能有假 肯定;链式作业无意间发生的情况 报告.

Warning: The chained assignment warnings / exceptions are aiming to inform the user of a possibly invalid assignment. There may be false positives; situations where a chained assignment is inadvertantly reported.

要为单个数据框关闭SettingWithCopyWarning，请使用

To turn off SettingWithCopyWarning for a single dataframe, use

df.is_copy = False

要完全关闭链接的分配警告，请使用

To turn off chained assignment warnings altogether, use

options.mode.chained_assignment = None

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