python - SettingWithCopyWarning，即使在使用 loc (?)

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在我不希望出现的情况下，我会收到 SettingWithCopyWarning 错误:

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

为什么情况 1 和情况 2 会发生？

最佳答案

在情况 1 中，df['A'] 创建了 df 的副本。正如 Pandas documentation 所解释的那样，这可能会在链接时导致意外结果，因此会发出警告。情况 2 看起来是正确的，但可能会出现误报:

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.

要为单个数据帧关闭 SettingWithCopyWarning，请使用