问题
Trying to create a new column in the netc df but i get the warning
netc["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
C:\Anaconda\lib\site-packages\ipykernel\__main__.py:1: SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead
whats the proper way to create a field in the newer version of Pandas to avoid getting the warning?
pd.__version__
Out[45]:
u'0.19.2+0.g825876c.dirty'
回答1:
As it says in the error, try using .loc[row_indexer,col_indexer] to create the new column.
netc.loc[:,"DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM.
Notes
By the Pandas Indexing Docs your code should work.
netc["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
gets translated to
netc.__setitem__('DeltaAMPP', netc.LOAD_AM - netc.VPP12_AM)
Which should have predictable behaviour. The SettingWithCopyWarning is only there to warn users of unexpected behaviour during chained assignment (which is not what you're doing). However, as mentioned in the docs,
Sometimes a
SettingWithCopywarning will arise at times when there’s no obvious chained indexing going on. These are the bugs thatSettingWithCopyis designed to catch! Pandas is probably trying to warn you that you’ve done this:
The docs then go on to give an example of when one might get that error even when it's not expected. So I can't tell why that's happening without more context.
回答2:
Your example is incomplete, as it doesn't show where netc comes from. It is likely that netc itself is the product of slicing, and as such Pandas cannot make guarantees that it isn't a view or a copy.
For example, if you're doing this:
netc = netb[netb["DeltaAMPP"] == 0]
netc["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
then Pandas wouldn't know if netc is a view or a copy. If it were a one-liner, it would effectively be like this:
netb[netb["DeltaAMPP"] == 0]["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
where you can see the double indexing more clearly.
If you want to make netc separate from netb, one possible remedy might be to force a copy in the first line (the loc is to make sure we're not copying two times), like:
netc = netb.loc[netb["DeltaAMPP"] == 0].copy()
If, on the other hand, you want to have netb modified with the new column, you may do:
netb.loc[netb["DeltaAMPP"] == 0, "DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
回答3:
You need to reset_index when you will create column especially if you have filtered on specific values... then you don't need to use .loc[row_indexer,col_indexer]
netc.reset_index(drop=True, inplace=True)
netc["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM
Then it should work :)
来源：https://stackoverflow.com/questions/42379818/correct-way-to-set-new-column-in-pandas-dataframe-to-avoid-settingwithcopywarnin