即使使用loc(?)，也可以使用SettingWithCopyWarning
问题说明
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?
正确答案
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
这篇好文章是转载于：学新通技术网
