pandas SettingWithCopyWarning，即使使用 loc (?)
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SettingWithCopyWarning, even when using loc (?)
提问by Amelio Vazquez-Reina
I get SettingWithCopyWarningerrors in cases where I would not expect them:
SettingWithCopyWarning在我不期望它们的情况下，我会收到错误：
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
and
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
Why does it happen in case 1 and 2?
为什么会在情况 1 和 2 中发生？
回答by kxsong
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:
在情况 1 中，df['A']创建df. 正如Pandas 文档所解释的那样，这可能会在链接时导致意外结果，因此会引发警告。情况 2 看起来是正确的，但也可能出现误报：
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.
警告：链式分配警告/异常旨在通知用户可能无效的分配。可能存在误报；无意中报告了链式分配的情况。
To turn off SettingWithCopyWarningfor a single dataframe, use
要关闭SettingWithCopyWarning单个数据帧，请使用
df.is_copy = False
To turn off chained assignment warnings altogether, use
要完全关闭链式分配警告，请使用
options.mode.chained_assignment = None
回答by Charlie Haley
Another solution that should suppress the warning:
另一个应该抑制警告的解决方案：
df = df.copy()
df['B'] = df['A']/25
df['B'] = df['A']/50
