Correct way to set new column in pandas DataFrame to avoid SettingWithCopyWarning
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 thatSettingWithCopy
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.
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
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 :)