python - Correct way to set new column in pandas DataFrame to avoid SettingWithCopyWarning

Question

Trying to create a new column in the netc df but i get the warning

netc["DeltaAMPP"] = netc.LOAD_AM - netc.VPP12_AM

C:Anacondalibsite-packagesipykernel\__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'

question from:https://stackoverflow.com/questions/42379818/correct-way-to-set-new-column-in-pandas-dataframe-to-avoid-settingwithcopywarnin

Answer 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 SettingWithCopy warning will arise at times when there’s no obvious chained indexing going on. These are the bugs that SettingWithCopy is 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.